Valve overlap is the period during the valve timing where both the intake and exhaust valves are open. Occurring towards the end of the exhaust stroke, the intake valves are opened just before all the exhaust gases are released, providing more time for the intake air to enter the engine.
Valve overlap is necessary because as the engine rotates faster, the amount of time spent in the intake stroke decreases. This means the intake valve has to open sooner to give time for the intake air to accelerate and flow into the cylinders. It is because of this that fixed camshaft profiles, which control valve timing, are only perfect at a certain engine speed. Variable valve timing was invented to solve this problem and help produce the maximum amount of performance possible at all engine speeds.