Pullets squat by lowering their bodies, tucking their heads down, and slightly spreading their wings away from their bodies. Some also will stamp their feet. This signifies they are ready to be mounted by the rooster.
When they do it for their people, it means they're indicating submission.
Generally, when they start doing this, they are within a month of laying. Adult hens may continue to do it throughout their lives. Or not.
Some hens won't squat, ever, even for a rooster. Or they may, now and then, when they feel like being submissive.
A gentleman rooster will accept a hen's refusal and not mount her. Young roosters are so charged with testosterone they don't care, and will mount a pullet or hen anyway. That causes a whole lot of chasing, feather pulling, and squawking.