A steady run right at the point where your body clears lactate as fast as it produces it — 'comfortably hard'.
Threshold work is the most efficient hard session in endurance training because it lifts precisely the speed you can hold for a long time — which directly drives race performance from 5 km to the marathon. The pace roughly matches what you could sustain in a race lasting an hour; on the talk test, short sentences yes, conversation no. Typical formats are 20–40 minutes continuous or blocks of 3 × 10 minutes with short jog recoveries. The usual mistake is running it too fast — then it becomes an interval session, recovery takes longer and the intended stimulus is lost. Rule of thumb: if you're completely wrecked afterwards, it was too fast.