From Halopedia, the Halo wiki
A Propane tank is a human storage device for the chemical propane. Propane can be used for cooking, light, fuel for certain vehicles, and heating. Propane tanks, when shot, fly forwards and explode. These explosions are powerful enough to harm or kill anyone nearby.
Along with power to harm and kill beings, Propane Tanks can also cause minor or severe damage to vehicles, depending on how close they are, and the amount of damage the vehicle can take.