Chamber blocking is a semi-hidden combat mechanic added in Mount&Blade: Warband. It is the act of blocking an incoming melee attack by meeting it with your own weapon or even your bare fists as you ready, or "chamber", any melee attack.
The mechanics are fairly simple. As you prepare your weapon to attack, it can stop other attacks that are being swung or thrust from the same direction. The sole exception is that both overhead and thrust attacks can be blocked by preparing an overhead attack of your own. Even if a weapon is normally unable to block, it can be used to chamber block as long as it can perform an attack to meet its opposite.
While considerably more dangerous, couched lances can also be blocked by chambering in the same fashion as fending off a normal thrusting attack. This is the only way to block a couched lance without sacrificing the durability of a shield, or the life of a horse or ally.