When you holler, you yell a little too loudly and abruptly. A kid stuck in a tree might holler for help until someone shows up to help him climb down.
Holler is an informal verb, useful for those times you call out or shout. Your teacher might holler at the class if she gets angry enough, and you might holler at your sister to get out of your room at least once a day. A holler can also be a dip or depression in the land, a hollow. The word dates from the 17th century, a colloquial form of hollo, a variation on hello.