Containment describes the blockading of an opponent's base. This is to make it easier for a player to create expansion bases and claim additional resources while denying the opponent the same. The crippling of the opponent's economy is a long-term result of a successful containment. Contains also typically prevent enemy scouts from leaving their base, keeping the contained player in the dark about his opponent's army makeup and position. Observers however can bypass most containments with ease due their small size and permanent cloaking.
A large proportion of a player's army may be deployed to contain the opponent's base, and may be fortified like any defensive line.