A squish area is a part of the combustion chamber that is made to very nearly come into contact with the piston when the piston is at top dead centre. There’s usually less than .030" clearance in these areas in such cases. What they do is cause the combusting gasses to rapidly travel towards the center of the combustion chamber and increase turbulence, thus increasing the combustion rate. By dropping the C/R with a thicker gasket, the squish area becomes not as effective, and you will lose some power, among other things.