Scr threshold voltage
It's simply the minimum gate voltage required to trigger the SCR. It is actually a current driven device, like a BJT (it's build much like a BJT in fact) but it also need a minimum voltage to trip (exactly like the base junction).
The gist of it is simply to say, don't try to trigger an SCR with 0.2V (even with the required 10mA or so) because it needs more to bias the junction.