A string follow bow means that the limbs on an unstrung bow are slightly curved back towards the shooter, as opposed to a recurve, where the limbs are curved away from the shooter, sometimes radically. String follow occurs naturally over time as a self bow that starts out as a straight limbed longbow, or even one that is slightly reflexed, is used, if it is not backed with fiberglass. Some self bows are made with string follow. Once string follow gets too pronounced, it is time to retire the bow.
A small amount of string follow reduces hand shock and seems to make the bow smoother drawing. The tradeoff is that any amount of string follow also reduces the bow's performance.