From Halopedia, the Halo wiki
A Mortar is any weapon or class of weapon that utilizes the firing of a projectile or other device in an arc-like fashion at a target. A mortar is also a classic indirect-fire weapon, and is generally used to support ground troops from a distance.
Traditionally, a mortar is a pipe in which a round (high explosive, phosphorous, or flare) fitted with a self-detonating charge is dropped in, and a firing mechanism at the bottom of the pipe launches the round in an arc-like trajectory.
The paradox of mortar weapons is that all projectile weapons are limited to arc-traveling projectiles, except the mortar is a classification of which that arc is incredibly profound. This arc allows mortars to fire over obstacles that may stop normal weapons.
The only mortars in Halo that are mentioned are of Covenant make: the two examples are the standard Wraith and Revenant's plasma mortar and the Fuel Rod Gun from Halo: Combat Evolved multiplayer. There are also portable plasma mortars that Grunts set up near the core of Onyx, though these were never fired because their crews were all killed.
Although the Surface-to-Air Missile Launcher is commonly believed to be a mortar gun, it actually fires anti-air surface-to-air missiles (AA-SAMS) instead. Mortar weapons are usually heavy weapons.