Blademail:

Blade Mail is an item that reflects damage back at the source for a short time after activation. It is used to punish enemies who deal large bursts of damage, especially when they focus a single hero or attack into a tanky frontliner.

In Dota 2, Blade Mail matters because it can turn aggressive spells and attacks into a risk for the caster, forcing opponents to time their damage more carefully or disable the holder first.

Related: Armor (Kevlar), Burst Damage