1) (noun) a mistake, usually a small one, but humorously a large mistake.
2) (noun) a small injury such as a cut or a bruise, used for children but humorously used for adults as well.
3) (verb; noun) the act of defecation (going poo), or the poo itself.
4) (adjective) general unattractiveness, lack of quality or badness.
Because of it is a relatively simple and humorous sounding word, it may have many more colloquial meanings within families, friends or groups.
2) Timmy fell off his bike and got a boo boo, he ran home to mommy so she could kiss it and make it better.
3) "Lets stop at the next restroom we find. I gotta go boo boo"
"I made a stinky boo boo!"
4) "Have you seen Duane's new car?" "Haha yea, that shit is boo boo!"