“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 say this as you are talking with a friend about the song.
It's pretty catchy.
You use "pretty" before an adjective to show how much of that quality something has.
"Pretty ___" is somewhere between "kind of " and "really". It basically means "a little more than you expected". For example:
This soup is actually pretty good.
I think I have a pretty good shot at getting the job.
That song is pretty catchy.
A "catchy" song is one that's easy to remember. It's also hard to forget.