What is squish in a 2 stroke engine?

Putting it simply as the piston moves upwards on the compression stroke the fuel / oil / air mixture is compressed from a large volume into the much smaller area between the top of the piston and the cylinder head as it reaches TDC. The volume of this area is the squish volume. In other Words it has been "squished". The velocity and heat generated in this process combined with the geometry of the head and piston affects the combustion process as it is pushed towards the spark etc