Shirts are usually long enough to hang below the top of your pants. When this happens, you have two choices: you can put your shirt inside of your pants, or outside. Putting it inside is called "tucking your shirt in":

Hey, tuck your shirt in.

Putting it outside is called "leaving your shirt untucked".

I think it looks better if you leave it untucked.

If your shirt is coming out of your waistline just a little, you can call it "untucked".