Pajarito means little bird so it can be used to describe a small creature of the feathered variety.
- Los pajaritos ya abandonaron el nido.
The baby birds have already flown the nest.
It can also be used in the same way as in English when you want to explain that you heard a secret.
- Un pajarito me dijo que estás embarazada. ¡Felicitaciones!
A little bird told me you're pregnant! Congratulations!
It is also one of many slang words for the male genitalia, and one of the least offensive and more usually used with children, translating as weenie or willy (depending on whether you are from US or UK).
And most usefully at this time of year, is a great way to complain about the cold.
- ¡Estoy pajarito!:
I'm freezing to death!.