Generally speaking the role of a back player is to keep his front guys asses in the game for as long as possible. Front players normally dont have a good view of whats going on all over the field due to them playing low bunkers and so are open to being mugged unless someone is watching over them.
Backs are the eyes of the front player and stop them getting mugged out by controlling the 'zone' assigned to them for any particular game. Controlling a zone means keeping players on the opposing team in behind their bunkers (by chucking loadsa paint)so they cant come out and rain a stream of paint on your front guy as he manoevers into key bunkers. Controlling the zone effectively also means the opposition wont be able to move from bunker to bunker in your zone without running into your paint (hopefully)! You should never break (take your eyes off) your zone, even for a second without letting your front guy know you are about to do it, because in doing so you are leaving him unprotected. Communication is therefore vital, as Bunka Monkey says. If your backs and mids have the field locked up by using zones then the front guy(s) can go for eliminations. Teamwork is the key to success, fieldwalking together and drawing out the field on squared paper helps you understand where your zone will be... but look out, cos it changes for every field!!