This phrase is frequently heard during the ensuing lulz and drama of an alleged chan raid.
When uttering the phrase, all caps is necessary. Otherwise, the lulz of the phrase is lost, and you must get raped. A less funny version, "BRBFEDS," may be used as well, however the true inner meaning might be lost.
Person two: I haxed yer daterbase
Person one: orly?
Person two: BRBFBI
"Just 15. Oh shi - BRB FBI."