"Kai" actually refers to "kai nigahein" (several glances).
The song is about the overwhelming power of love, and the way it captures your attention and leads you away from everything else.
So, the phrase "Najaane ek nigaha mein kya le gaya Kai" essentially means:
* "I don't know what took me away, but it was several glances."
Let me know if you have any other questions or want to explore more about the song!