As above, the forward voltage is the voltage required to push any current at all(*) through the LED. Also called Vf (the f should be subscript but many formatting options prevent that). For all LEDs, this voltage increases with current (otherwise you could have a runaway LED that drew more and more power). There’s other factors in play as I recall, like temperature. But it’s the fundamentals of why some LEDs with higher Vf (like XPL-Hi) are fine with single-cell direct drive, and others are not (Osram 1mm would fry itself).
*: there is leakage current, but this is valid for our purposes