What is overlapping of valves?

Valve overlap occurs in a four-stroke engine at the end of the Exhaust stroke and at the beginning of the Induction stroke; and is when both the exhaust and inlet valves are open at the same time.

At the end of the Exhaust stroke, the Exhaust valve is closing and the Inlet valve is starting to open in preparation for the induction of fuel and air into the cylinder.

It is common knowledge that in vehicles that cannot alter the valve timing, the Inlet valve opens approx 6 degrees before top-dead-centre and the Exhaust valve closes at 9 degrees after top-dead-centre