piston to valve clearance.

is the thousand he mentioning is space between the valves and piston head?

No, it's between the valves and the top of the piston.

On most engines, the valves will open far enough to occupy the same space as the piston as it is close to, and at, TDC. The greater the valve lift and/or duration, the greater the chance they will hit each other, because the valve(s) will spend more time off the seats.

This is checked so the pistons can have material removed, if required, but most aftermarket/performance pistons already have material removed to give more clearance.