Umm, it could not be
simpler. "
Holla" means "
holler"... which means "to shout".
If you want to be all ghetto about it then you can use it somewhat inappropriately within a conversation to the effect of: "please reply", or "talk to me".
Colloquialism: "Hey dogg, I know dat you be cool wit' THOSE cats, why don't you holla at yo' boyz?!"
Text Message: "
wut b gwanin? i b
cotchin in da toilet. u goin to da club l8r? tb/holla back xxx"