In laymans terms, you know how when you push really gently on the analog stick you have that looseness and then you hit a point where it tightens up and has a bit of resistance? Usually when you hit that spot your reticle/crosshair in game will begin to move. However if you have a deadzone, you must push the analog even further before you get that movement. What this does is give you less precision and comfort with the aiming as you have to make larger rotations (for example) to achieve small ones. So if you are trying to aim at a bad guy in any game and lets say you want to go for a headshot, if you push gently it might not respond at all. If you push a bit farther, you might end up over-shooting.