It makes the character say "Sapped" with the skulls on each side when a rogue saps him.

Optimized for performance so it isn't checking every single combat log entry for saps when in a scenario, dungeon, raid, or rested xp area where large populations like to congregate.

There's various weakauras, tellmewhens, and even addOns called "Sapped!" and "Say Sapped" that do the same thing, but some people are having trouble finding optimized and up to date versions of them.