The Full Form of CQB is Close Quarters Battle.

Close-quarters combat (CQC) or close-quarters battle (CQB) is a tactical concept that involves a physical confrontation between several combatants at very short range. It can take place between military units, police/corrections officers and criminals, and in other similar scenarios. In warfare, it usually consists of small units or teams engaging the enemy with personal weapons within a distance up to 100 metres (110 yd), from proximity hand-to-hand combat to close-quarter target negotiation with (usually rapid-firing) firearms. In the typical close combat scenario, the attackers try a very fast, violent takeover of a vehicle or structure controlled by the defenders, who usually have no easy way to withdraw. Because enemies, hostages/civilians, and fellow operators can be closely intermingled, close-quarters combat demands a rapid assault and a precise application of lethal force. The operators need great proficiency with their weapons, and the ability to make split-second decisions in order to minimize accidental casualties