The phrase «Μηδείς αγεωμέτρητος εισίτω μοι την θύρα» (or «Μηδείς αγεωμέτρητος εισίτω μοι την στέγη») is a phrase that supposedly was on the threshold of Plato's Academy and translates to: "do not let anyone ignorant of geometry through my door" (or under my roof). See for instance this discussion https://www.plato-dialogues.org/faq/faq009.htm
The phrase in AMS' logo translates to "Do not be ignorant geometry".
Edit: The above translation is not correct. The word "εισίτω" is a form of the verb "εἴσειμι" which means "to come into" (wiktionary). I initially thought that it was a form of "ειμί" which means "to be".
So finally the AMS' logo says: "Do not enter if you don't know geometry" or "You will not enter if you don't know geometry".