Wall hacks are basically ESP hack and another very common cheat used in the Battlefield 4 multiplayer. Such a hack will simply show you the enemies even if they are behind a wall, allowing you to kill them easily even though you are not supposed to know their position. These kinds of hacks, again, are easily detected by anone watching you play and should be used with caution. If people see you killing you headshot someone through a wall once, they might categorize that as luck, but if they see you constantly giving people wall-headshots, then they will know that you are a hacker and ban you from their server and maybe even report you to EA and that is not what you want.