Valve bounce is a condition that describes the action of the valve hitting the seat at high revs and bouncing of the seat again before being controlled by the camshaft.
The valve spring is designed to close the valve onto the seat and hold the valve in that position until the action of the camshaft is positioned to open it again. If the spring is below specifications the spring will not keep the valve seated and valve bounce will occur. In modified engines valve springs have to be selected with higher specifications to cope with the higher revs developed by the modified engine. valves can also bounce simply because the engine is being over-revved due to driving conditions. (sustained full throttle operation in lower gears or free revving.)