The Mortar launches shells into the air that burst and send a deadly explosive onto enemies, causing an area-of-effect damage. It's the go to weapon for combating large groups of enemies, especially if they are at long range.
When operating the Mortar, you must first set the range, then fire. A meter appears in the middle of the screen to indicate the approximate distance the round will travel before it detonates. The longer you hold the Fire button, the further the round goes.
The Mortar first appeared in Gears of War 2.