I am glad you asked that. I always assumed "squish" had something to do with the soldering wire being squished when I saw pictures of the clearance checks. But it made me use google and I found a Wikipedia article about squish and it is very interesting.
That is how you read where the squish is. The squish is to increase velocity, improving burn. To much squish and you quench the flame kernel though or worse, start kissing the head when heat expansion gets going as the engine warms up.