5yo endocrinologist said she had slight low posterior hairline, what does this mean?

Reevaluation needed: Solitary low posterior neck hairline does not mean anything, but if present along with other signs, like short stature, short neck, widely spaced nipples, increased carrying angle at the elbows, heart murmur etc can be a syndrome, mostly genetic, eg: Turners Syndrome. She should be reevaluated, if any of these signs or other signs are present, if not wait until she reaches her pubertal age.