Damage

From Supreme Commander Wiki

Damage refers to when weapons reduce a unit's (or shield's) hit points. If a unit's hit points are reduced to 0, the unit is destroyed. If a shield's hit points are reduced to 0, the shield collapses and must recharge.

There are various kinds of damage. The simple kind affects a single target unit. Some weapons damage all units within a radius of an explosion. Some weapons do damage over time. Some weapons only damage enemy units, even if friendly units are within the area of effect.