A public
greeting, usually on a
call-in radio show, in a
newsletter of some kind, or another medium in which strangers are likely to notice the
shout out.
Many "urban" (hip-hop) radio stations in the US have a scheduled program for listeners to phone the station, go on the air, and give a shout out to their friends homies. (Thanks to PureDoxyk for the concise, if publicly humiliating, language tips.)
This is a shout out to all y'all n**ers on E2... keep it real!