The song's engine is a paradox, and it is stated twice before the song is a minute old. Khud ko main haar gaya: I lost myself. Tumko, tumko main jeeta hoon: and you, you, I win. The paradox is that the two sentences are the same event. Losing the self and winning the other are not two things that happened, they are one thing described from both ends. The song spends the rest of its length explaining that it is not a joke or a riddle, just the exact shape of the thing.
The opening passage begins with the smallest possible sentences, and that is the design. Tum ho. Tum ho paas mere. Saath mere ho. You are. You are near me. You are with me. Urdu's great love poetry usually arrives in metaphors. This song arrives in facts, and the facts are the intimacy: there is nothing to compare you to, you are simply present, and the present tense is doing the work.
The same passage then makes the song's most delicate move. Jitna mehsoos karu tumko, utna hi paa bhi loon: the more I feel you, the more I have you. The claim is that sensing and possessing are the same activity. Most love songs worry about holding on. This one says holding on happens automatically the more he lets himself feel, which is the calmest possible answer to the fear of loss.
The third verse is the memory of how it began. Kahin se, kahin ko bhi, aao bewajah chale: from somewhere, toward somewhere, let's walk, without a reason. The meeting without a why, bewajah, is being remembered as the purest thing that ever happened to him, and the last line of the verse says what the meeting did: bandishein na rahi koi baaki. No bonds left. The word for the old ties, bandishein, is the word for shackles, so the song is saying the meeting took the shackles off.
The last verse is the boast of the whole song, and it is a quiet boast. Kis tarah chheenga, aa, mujhse yeh jahan tumhe: how will the world ever snatch you from me. The answer is in the next line: tum bhi ho main. You are also me. You cannot be taken from me because you are no longer entirely outside me. And after that, one sentence of perfect freedom: kya fikar ab hamein. What worry is left to us now. The song ends where it began, on the paradox, fully believing it.
Mohit Chauhan's voice made the song a standard within a year of the film. The film version, with Kavita Krishnamurthy sharing the last passage, is the one people mean.