The Wikipedia entry is hard to grok:
Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile. This reduces engine efficiency and performance and potentially increases engine emissions. There is also a significant risk of severe engine damage due to valve spring damage and/or pistons contacting the valves.
Can someone translate this into something more understandable?
Diagrams, videos and animations are a welcome bonus.