I swear by this city,  and (O Holy Prophet,(SM) you are going to be allowed to fight in this city  and by the father and that which he begot,  Indeed We have created man (to live) in hard struggle.  Does he think that no one has power over him?  He says, "I have spent a lot of wealth."  Does he think that no one has seen him?  Did We not make for him two eyes,  and a tongue and two lips,  and showed him the two ways?  Yet he did not make his way through the steep course,  And what may let you know what the steep course is?  (It is) freeing of the neck of a slave,  or giving food in a day of hunger  to an orphan near of kin,  or to a needy person lying in dust  then he did not join those who believe and advise each other to be patient and advise each other to be merciful.  Those are the People of the Right Hand. (i.e.their books of deeds will be given in their right hands)  As for those who reject Our verses, they are the People of the Left Hand. (i.e. their books of deeds will be given in their left hand.)  Upon them will be the Fire, enveloping them with closed exits. 
لَا أُقْسِمُ بِهَـٰذَا الْبَلَدِ (I swear by this city...90:1). The particle la ('no') in the beginning of this sentence has no meaning here. Such particles in Arabic language are commonplace. However, the more appropriate view is that when a Surah begins with a la (no) followed by an oath, it was revealed in refutation to a false assumption of the opponents. In other words, Allah is saying, 'No, what you [the unbelievers] are saying or thinking is not correct, but the truth is what We are swearing about...'. The word al-balad 'the City' refers to the Holy City of Makkah, as in Surah At-Tin [95:3] where Allah swears an oath 'by this peaceful City [of Makkah] وَهَـٰذَا الْبَلَدِ الْأَمِينِ The adjective attached to the 'City' is 'peaceful'. This shows the superiority and honour of Makkah over other cities. Sayyidna 'Abdullah Ibn 'Adiyy رضي الله عنه reports that when the Holy Prophet of Allah (SM) was migrating from Makkah to Madinah, he addressed the city of Makkah and said: "By Allah, you are dearer to Allah than the entire earth. If I was not forced out of this place, I would never have abandoned you." [Transmitted by Tirmidhi and Ibn Majah vide Mazhari].
وَأَنتَ حِلٌّ بِهَـٰذَا الْبَلَدِ (and [O Holy Prophet (SM)] you are going to be allowed to fight in this city....90:2). The word hillun bears two possibilities:  It could be derived from hulul which signifies to reside in or to descend on some place. In this sense, hillun signifies 'a dweller or resident' and the verse purports to say that the city of Makkah itself is sacred, and since Holy Prophet Muhammad is the inhabitant of this city, it adds to the sanctity, honour and glory of the place. Thus, on account of the Holy Prophet's (SM) residing in that city its honour and sanctity has been enhanced and augmented. The second possibility is that it is derived from hillatun which means 'the thing the doing of which is lawful'. From this point of view, hillun could signify one of two things: One that the pagans of Makkah consider it lawful to do the Holy Prophet any harm, even to kill him, in this City of Makkah which is so sacred that the doing of harm to a living creature in its precincts, not to say of killing it or hunting it, is strictly forbidden even according to their own belief system . Secondly, this may be a prophecy that the city of Makkah is going to be made lawful for the Holy Prophet in the sense that fighting in it will be allowed for him for a specified time, and He will alight in this City of Makkah, as it happened on the occasion of the Conquest of Makkah. On this occasion, all the injunctions and ordinances of the haram were suspended for one day, and it was made lawful to kill the disbelievers. Mazhari cites three possible meanings, and says that all three of them are equally possible.
وَوَالِدٍ وَمَا وَلَدَ (and by the father and that which he begot,...90:3) The word walid refers to Holy Prophet 'Adam عليه السلام, the father of mankind, and the phrase d;d 'that which he begot' refers to his children from the inception of the world to the end of the world. Thus this phrase swears an oath by Holy Prophet 'Adam عليه السلام and all his children. The subject of the oath follows next, thus:
لَقَدْ خَلَقْنَا الْإِنسَانَ فِي كَبَدٍ (Indeed We have created man [to live] in hard struggle...90:4) The word kabad means 'labour, 'toil' or 'difficulty'. The verse purports to say that man's life is a series of hard and toilsome works. Ibn 'Abbas رضي الله عنه says: 'Man was conceived and held in his mother's womb. The mother bore the pangs of birth. The hardship of sucking the mother's milk and the difficulty of weaning. This is followed by seeking livelihood and other necessities of life with hardship. Then he endures hardships and long-suffering of old age, death, grave, resurrection, accountability of deeds before Allah, reward and punishment.' These difficulties and hardships are not confined to man. Other animals too share them. Man has particularly been mentioned in this connection because of his intelligence. The more the power of a creature's intelligence, the higher the degree of his legal obligation. Lastly, the greatest difficulty and hardship would be borne at resurrection and life-after-death, when we will be required to give an account of the deeds we might have done throughout our life. Other species of animals will not be required to do Some scholars say that no creation suffers as much difficulties or hardships as human beings, despite the fact that his body is smaller and weaker than most other animals. Man's brainpower, however, is most powerful. Therefore, he has been specifically mentioned. Swearing an oath by Makkah, 'Adam and his children, Allah has made it plain that man has been created in difficulties and to endure hardships. This is a proof that man did not come into existence on his own but his Creator is an All-Powerful Being who has, in His wisdom, created every species of creation with specific predisposition and capacity of actions. If man had any part in his own creation, he would never have allowed such difficulties and hardships for himself. [Qurtubi].