A military mortar consists of an angled barrel attached to a baseplate, with a fixed firing pin at the bottom of the barrel. The operator drops in a mortar shell, which slides down the barrel and strikes the pin. This detonates an explosive charge, firing the shell out of the barrel towards its intended target. When the shell strikes the ground or another hard surface, it explodes.
Mortars are indirect fire weapons, which means that the projectile comes in from above rather than being fired directly at a target. This makes them relatively inaccurate weapons on the battlefield, requiring regular adjustments to walk subsequent shots into the desired area of effect. They are also extremely simple weapons, requiring only metal tubes and scrap to construct. They are especially popular with guerrilla fighters due to their portable nature, allowing a unit to deal out artillery-scale damage while remaining as mobile as infantry.
The first mortars were developed in the 15th century as siege weapons and got their name because the bulky, inaccurate devices resembled the stone bowls used by apothecaries. Mortars using the modern tube design came about in the 17th century and first saw widespread use in the Jacobin Uprising in the early 1700s.