To peep is to take a quick, secret glance at something. Kids might peep into the living room on Christmas morning to see if Santa came the night before.
You can peep through the blinds to see who's ringing your doorbell, or watch the sky, hoping the sun will peep (or suddenly appear) from behind the clouds so you can go to the beach as planned. Another kind of peep is the high chirping sound a chick or baby bird makes. This second meaning is imitative — in other words, the word peep sounds like the noise itself.