A glancing blow is caused when a melee attack fails to land properly. This is usually because an attack is misaligned, or the target is at the outer edge of the attack's range. In these cases, if the attack fails to hit the opponent squarely, they will fail to yield full damage.
All glancing blows, except one case, deal exactly half the damage that will be dealt with a solid hit.
The only exception to half damage rule is the Thunder Hammer/Daemon Maul with the Killing Blow perk, that causes tremendous overkill with 2000 damage on a solid hit, but is reduced to 150 damage on a glancing blow to allow imperfect hits to be survivable.