KNOW WHY DEPRESSION IS PROVIDED ON PISTON HEAD !!

At the end of compression stroke the piston is at the top dead center.During 
compression and power stroke both valves are closed.But in exhaust stroke exhaust valve is opened and piston moves from Bottom Dead Center to Top Dead Center.Similarly,in suction stroke intake valve is opened and piston moves from Top Dead Center to Bottom Dead Center.
To maintain high compression ratio, very less space is left on the top of the piston when it is on the top dead centre. 

So when valves open they can actually hit the piston if depression is not there. That cut in picture is called valve relief pocket that prevents valve from hitting piston.