Va is the speed in which above it, it is possible to damage the aircraft structure before a positive stall is achieved with positive loading.
You can find this on a V-G diagram.
**
Two identical aircraft are flying at the same speed.
The heavier aircraft must maintain a higher AoA to maintain its straight and level flight path, compared to the lighter aircraft, which has a lower AoA
Hence for the lighter aircraft to reach the critical AoA (same for both aircraft) you need to pull harder to achieve the stall, in which some cases, you may exceed the maximum allowable positive G-Load factor for the airframe.
Simply put, a heavier aircraft is easier to stall than a lighter one.
I'm sure if you search the forums there are multiple threads on this topic, and many different forms of explainations