"get back to -something-"

probably from old colonial / racist exclamations of wanting economic / refugee immigrants of former slave classes to "get back to africa (and stop stealing our jobs)" etc. In this case usually an expression of the poster informing the subject that they are in the wrong board section (or wrong board altogether) and where they should go to ("take" their wrong-subject post "back" to).

e.g. someone within the 4chan lot puts a pornographic post on a board about kittens... incensed felophile orders them to "gb2/h/ !!!1" and might well request "modz" to have them "b&" (b-and, banned)

might also be written in this case as a more simple "gb2h" without the slashes... if they posted something that was not allowed on the board but allowed on another (e.g. iichan not 4chan) there would be "gb2/iichan".... and for things that are just plain wrong and have no home, there's the universal GTFO (get the f--- out)

easy enough when you know how ;)