To greatly insult someone verbally (and sometimes physically) about themselves, situations they've been in, their family, friends, or anything under the sun. It is more of a competitive thing mostly used to see who can make the other person feel stupid. Types of jokes used are jokes about a person they look like, objects they look or smell like, a quote from a song, book, movie etc.. that they are relevant to, their clothing, etc. Most people use these to have a laugh, some people use it to piss the other person off (leading to fights).
There are certain words to use after every refernce such as
or anything else having to do with the previous heat. Those are the more commonly used ones though.
Bob: Kevin you heatin nigga?
Kevin: get yo big bird lookin ass boi.
Bob: aaah hell yea im finna get yo Barry White lookin ass.
Kevin: hell yea look like this nigga got lined up wit a blockbuster card.
Bob: i know this nigga not tryin to heat wit that tight ass shirt on, you can see that nigga heart beat through that shit.
Kevin: man get yo big Amistad face ass big "LET US FREEE" toe ass boi.
(everyone that was watching laughs at Bob because Kevin made him feel stupid with his last heat.)