Mass in Doom engine games is a little different than the scientific definition of mass and the Doom engine has relatively simple physics that handle functions related to it. Every object in the game has a mass value and every projectile, hitscan attack and blast has a force value that pushes objects on impact, assuming the object is shootable, such as a monster, a player or a barrel. A projectile moves the object it hits to the direction it was going in before the impact. A blast moves the hit object away from the center of the blast. A hitscan attack moves the hit object away from the actor that made the attack. The higher an object's mass is, the less the object will move when hit. The lesser the mass, the more the object will move.
When mass is messed with using DeHackEd, it may have detrimental effects on gameplay. If an object's mass is negative, then it will be pulled towards the blast or attack. If the mass is changed to zero, then the game will instantly crash when it is hit. If the player's mass is set too low, the acceleration caused by a blast may speed him through a solid wall.
A player's mass also defines how much they slide when they stop moving.
Decorative objects such as dead bodies or items have a mass value as well, but they do not use it for anything as they cannot be moved in any way.