Va is the "A" point on the V-N diagram, that is, it's the point at which the aircraft should stall exactly at the design structural limit N1. Below that, you should theoretically stall first, above that, you're eating into the safety factors and ultimately can bend the aircraft.
However, it is all a little more complex than that. If you pitch-up aggressively, you can get to a higher AoA before you stall, so you could in theory and practice overstress at a lower speed - albeit through mishandling. (This is called "unsteady aerodynamics" and is a bit of a black art.)
A further issue is that the primary flight controls are designed for full deflection only up to Va, above that the maximum deflection tapers off, but they are also designed for at-least 1/3 deflection at Vne, which is why you're told not to use more than 1/3 deflection above Va.
So, in practice, you are pretty safe from anything except very rapid control inputs, particularly in pitch, below Va. But as JT says, smooth manoeuvring is a good thing regardless !
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