The term is imitation of the sound of someone crying or sobbing. It is often seen written, like in cartoon speech balloons. Less often, a person may say it aloud in an affected way to convey sadness or disappointment. Can be used sarcastically.
"Boo hoo," cried the little girl after she fell off her bike.
- OR -
"Boo hoo!" mocked the others, "Don't be such a crybaby."