it usually happens during high rpm , pretty much the valve lose contact with your cam, and they dont fully close . so pretty much the force of your comubustion pressure overcomes your valve springs due to higher rpms.
Valve float is just as @ DTARS stated, this is when the cam and valve train start to lose contact with each other. There can be several causes, one being too steep of a lobe profile or ramp rates that far exceed the spring pressure and or springs that are worn and no longer have the same capacity that they were initially intended to have. Typically this is a higher rpm problem and with either weak springs or springs that are not suited for your valve lift/profile is where you will run into this. This is the reason that everyone suggests changing the valve springs with a GT40p head swap for example.