“It's pretty catchy.”

You heard a new song a few days ago, and since then you've been thinking about it a lot. You are talking with a friend about this song, and you say:

It's pretty catchy.

pretty (adjective)

You use "pretty" before an adjective to show how much of that quality something has. It basically means "a little more than you expected". For example:

This soup is actually pretty good.

You were pretty drunk the other night.

a catchy song

A "catchy" song is one that's easy to remember. It's also hard to forget.