Questions & Answers about Settu bílbeltið á þig áður en við förum.
Settu is the singular imperative of setja (to put, place, set).
So the sentence is directed at one person and means something like a command or instruction: Put on ...
Compare:
- setja = to put
- settu = put! (said to one person)
- setjið = put! (said to more than one person, or used politely)
Icelandic often does not use a possessive like your when ownership is already obvious from the context.
Here, á þig (on you) makes it clear whose seat belt is meant, so bílbeltið literally means the seat belt, but in natural English we often translate it as your seat belt.
This is very common with things connected to the body, clothing, and personal items when the owner is obvious.