Generally used as the maximum weight of a rocket for a specific motor and delay. This is based on the thrust curve and average thrust of a motor. The main objective is to have the rocket traveling at a velocity where the aerodynamic forces produced by the fins are suffficient to keep the rocket pointed in the intended direction. When it is windy the angle of attack of the fins is not zero and under some circumstances where the thrust to weight ratio is low and the velocity is too low, the fins will stall and the rocket will do funny things. Underpowered rockets may pinwheel in the air in moderate crosswinds. A good rule of thumb is a 5:1 thrust to weight ratio on windless days, and a 10:1 ratio on a windy day. Also the longer the launch rod, the higher the allowable weight all other factors being equal. See also Stability and EMRR's Maximum Liftoff Weight Calculator