depends on what you want to use it for but it's a floating point value assigned to a vertex in max it can be used for affecting subdivision..... I think there are other modifiers that use it but I can't remember which ones at the moment
in general if you hear about weights it usually means that something will be associated with a number value, usually between 0 and 1. So if you have vertex A and vertex B, and you have Arm and Foot, if vertA has 0.2 weight to Arm and 0.8 weight to foot, it will go more towards the foot. And vertB has weights like Arm = 1…