What is WallHack?

I know, noob question but, What is WallHack?

I thought wall hack means you shoot threw the objects and wall
But its like glasswalls in the hacks i tried, if its just looking at the characters positions, its like a lame version of chams. I would like wall hack to mean, Shooting threw the walls! but...im not sure what wall hack is anymore -_-. People in game yell "Wall hacker" cause you kill them while behind an object, but in hacks wall hack just shows their positions, not shoot threw walls.

sry, but i wount tell you, its secret-- so boring