Checking for dilation

When a doctor checks for dilation or thinning, what is he or she feeling for? Is it like when you check to see if you're cervix is open? Like how suggested when you're trying to see if it's hard or soft when you're ovulating.
Is it your cervix that he or she is trying to see is open?
I'm not gonna try and check myself, I'm just curious. I haven't taken to Google yet and my appointment isn't till Tuesday or I'd be asking him how it all works.