"Chirp" is the sound that a bird makes. When parents teach their children animal sounds, they usually teach "Chirp!" as the sound that a bird makes:

A: What does the birdie say?

B: Chirp! Chirp!

Adults also use "chirp" to talk about bird sounds. Sometimes we use the form "chirping":

Do you hear that chirping? What kind of bird is that?

Some insects, like crickets, also "chirp":

The crickets are chirping pretty loudly tonight.