As you look at the round, you’ll notice 4 yellowish discs on the base of the round. These are propellant increments. By removing discs, you can vary the range of the round. There is a small charge built into the round itself. It vents through holes in the side of the round, into the mortar tube itself. That charge in turn ignites any propellant charges on the round. The fat part of the round forms a seal ahead of the burning propellant.
Rounds come with the maximum number of propellant charges already affixed, and they are removed as needed just before firing. After a shoot, excess propellant charges are burned- well away from the remaining ammo. The burn barrel is pretty cool to watch.