Warding is an intrinsic that sometimes causes the melee and ranged weapon attacks of summoned monsters against you to fail:
(Monster) tries to attack you, but flinches away.
The chance of the attack being prevented is based on the monster's magic resistance. Each attack forces a roll of the monster's MR vs. 1d60; if the monster's roll wins, the attack goes through as normal.
However, it will not prevent summoned monsters from casting spells or using abilities such as torment or hellfire against you. This makes it much more useful against such spells as Summon Horrible Things and Haunt than against Summon Greater Demon, for example.
Warding will not provide protection against monsters that are durably summoned.
Warding can be obtained through: