I'm not sure what is meant by a low-dropout regulator
A linear regulator requires some "overhead" voltage to work. A 7805 (one "standard" linear regulator) requires at least 7V in Vin to give a reliably regulated 5V out. This difference (7V - 5V = 2V) is the dropout voltage.
A low dropout (LDO) regulator's dropout voltage is less than the 2V of the 7805. Some are in the 400mV - 600mV range. I saw one description that called a regulator LDO when the dropout voltage was 1.4V - that's a bit high IMO, but it is less than 2...
-j