guess what?

bro do not fall for it. you think they are about get you with the most elaborate joke that you have never heard of but they are going to gut check you with the words"chicken butt" at this moment you must murder them because that joke is so unfunny that its funny.

(person 1 ) HEY GUESS WHAT BRO??
(person 2 aka the victim) what man?
(person 1) chicken butt.*starts laughing like the joker*

in all seriousness "guess what?" usually means that somebody is about to hit you with a joke or wants to tell you something