ESP in gaming language (mainly counterstrike games and mods, fps games) means Enemy Sound Position. It is an illegal program, cheat, which shows a square or dot or other sign on the position of other players (of opposite and/or the same team), and that symbols are visible through walls so user of esp can see where enemies are. This is kind of wallhack.
-thx man! this is sooo great esp! without it our team would lost this match!