The intensity above which blood lactate starts accumulating faster than the body can clear it, marking the edge of sustainable effort.
Below it, an effort can be held for a long time. Above it, the clock starts. For a trained client the threshold sits somewhere around 80 to 90 percent of maximum heart rate, and roughly at the pace they could hold for an hour.
It matters more than VO2 max for most endurance performance, because it decides what fraction of that ceiling can actually be used, and unlike VO2 max it is highly trainable.
Two tools raise it: a large base of easy zone 2 work, and threshold intervals, for example 4 x 8 minutes at a hard but controlled pace with 2 minutes easy between. One threshold session a week alongside easy volume is enough for a client who also lifts.