What did the dentist mean when he said all of this?

I had to go to the dentist today to get sealants, and they also took pictures of my teeth and then did an exam for the braces I should be getting in a few months. (My dentist is also an orthodontist). He kept calling out things to the nurse like "Crossbite on back teeth. 4 millimeter on top. 8 on bottom. 1 millimeter overbite". What did he mean?