material ID.. when you assign this number to an object in BF it will then react with that materials properties/habits.. when you shoot concrete it displays effects that look like you are shooting concrete.. when you drive on some ice.. you slide more than if you dirve onto something made of rubber.. just how you tell the game engine what kind of thing it is dealing with.
the matid is held within the collision mesh for an object which is a lower detail version of the object to optimise performance.. think of it like this.. you are in a space shooter game and launching a missile at earth.. the model of earth looks very detailed but really all the game needs to know is that it looks like a sphere.. so you see a highly detailed model of earth with a missile shooting at it.. but the game just sees a low detail, round object that the missile can hit.
When you throw a knife at a stratos it bounces off.. because the matid of a stratos collision mesh is Armour (45 in BF speak) but when you throw it at some wood (80).. it sticks..
This list is a very useful reference when you are making objects in BF so they behave in the right way in game.