Now, in the datasheet I can see this a bunch of times "If = 20mA". What exactly does this value mean and when should I take it into account?
The reason I'm asking is that I need to stick a bunch of these LEDs behind a BC547 NPN Transistor (datasheet) and as I understand it, I can only "run" up to 100mA through this transistor. I need to calculate my LEDs' circuit current so that I don't exceed this and damage the component.
I need to stick a bunch of these LEDs behind a BC547 NPN Transistor
The reason is their resistance varies wildly as you approach the sweet spot (usually 20mA). Trying to pick a resistor isn't easy. You can get big variance in brightness between LEDs.
This isn't the real story.
LEDs work by controlling the current, not the voltage.
I suspect that English is not fungus' native language, and that he meant "LEDs work best when the current is controlled, not the voltage".
In any case, diodes conduct current at every nonzero voltage, forward or reverse. There is no discontinuity in the applicable voltage/current relationship in either direction.
No, but there's a point on the curve where it suddenly goes exponential and it's usually very close to the "forward voltage" in the datasheet.