Suppression is a feature in The Last Stand: Dead Zone.
Survivors in cover can be suppressed by heavy gunfire and rendered unable to fight back. Taking gunfire whilst in cover 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 until their suppression meter empties, which it will do after a short period of time. How quickly survivors are suppressed depends on the suppression rating of the weapon used and the rating of the cover being fired at.
Survivors can be suppressed in one of two ways, either by being directly targeted by gunfire (in which case, only that specific survivor is suppressed) 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 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 in cover, as the infected lack both self-preservation instincts and enough intelligence to know how to use the environment to protect themselves from projectile weapons, while uninfected opponents outside of cover can simply be attacked directly.