Not to be confused with the weapon attachment.
Suppression is a feature in The Last Stand: Dead Zone.
Survivors can be suppressed by heavy gunfire and rendered unable to fight back. Taking gunfire causes a meter to build up, indicated by a yellow circle with an exclamation mark in it which fills up as they take fire. Once this meter is full, the suppressed survivor cannot attack and will have reduced grenade throwing range until their suppression meter empties, which it will do after a short period of time. Their range ring will also turn yellow as an indicator. Survivors can, however, heal while suppressed, if they have that ability.
How quickly survivors are suppressed depends on the suppression rating of the weapon used and the rating of the cover they're using, if applicable. Survivors outside of cover receive twice the suppression compared to those in cover. Survivors with higher suppression resistance receive less suppression than those with lower resistance.
Survivors can be suppressed in one of two ways, either by being directly targeted by gunfire (in which case, any survivor near the targeted survivor are suppressed as well) or by having the object they are using as cover being targeted by gunfire through the Suppression command (in which case, all survivors using that piece of cover and any survivor nearby are suppressed). The Suppression command must be specifically ordered by the player by holding down the Spacebar and clicking on the object that the player wants to suppress.
Suppression is only effective against uninfected opponents, as the infected lack both self-preservation instincts and enough intelligence to know how to use the environment to protect themselves from projectile weapons.