A "judgment-free zone" is a setting where friends turn a blind eye to petty social gaffes in the interest of promoting a laid back environment.
The term is often used to assure another person that they do not need to worry about behavior (their own or that of another) for which they just apologized.
Tim: "Sorry about what happened at the party last night. Do you think my stripping act in the kitchen freaked anyone out?"
Jon: "Hey, judgment-free zone, man."