The Land Mattress is the Allied equivalent of the German Nebelwerfer.
It is a 16-barreled rocket launcher which is crewed by 2 men. Each time it is clicked (if loaded) it will fire a barrage of 16 shots towards enemy lines. These vary in accuracy, so the land mattress is best used against large numbers of enemy troops.
To fire again, it must be reloaded, which is done by clicking on it. This will cost 1 TP. Note that you don't have to reload it directly after firing, but at any time. The land mattress must cool down for 15 seconds before firing again.
The Land Mattress is best when supported by M3 AT Guns or Shermans as its AT capability is relatively low. It's still good for AP, though it was an extremely powerful AT weapon when it was first introduced. The land mattress was nerfed in the next patch, which reduced its AT capability. Patch 2.2.7b upgraded its AP range to 200px (same as a grenade's).
This weapon is easily destroyed by AT weapons, and grenades can damage it and kill the crewmen, so it shouldn't be placed at the front line. A mechanic would come in handy to repair it if damaged.