Csection operative finding:...Very thin lower segment with only membranes covering... Any idea what that means?

Uterine rupture. When people have a c section the scar on the uterus can thin out. During labor this scar can rupture causing a life threatening situation. Your doctor is describing that your lower uterus was very think meaning you were at risk if you had tried to have a vaginal delivery. The doctor is also making a note in case you were wanting to attempt a vbac next time.