I'm sure this has been discussed elsewhere on pprune, and I seem to remember various subelties and debates to the issue.
However in simple terms Va can be considered as the speed at which, if you abruptly pull the elevator, the accelerated stalling speed coincides with the limit load factor. i.e at speeds below Va you will stall before overstressing, above Va you will overstress before stalling.
Consider a normal cat aeroplane with limit load factor 3.8g. The square root of 3.8 is 1.95, so Va is 1.95 x Vs (where Vs is the 1 g stall speed).
Since Vs varies with weight, so does Va