The lactate threshold (LT) is the exercise intensity at which lactate production exceeds clearance, causing blood lactate to rise exponentially. Training at or just below LT produces the largest adaptations in aerobic capacity.
Below LT: aerobic metabolism dominates, lactate is cleared as fast as it's produced. Above LT: anaerobic contribution increases, hydrogen ions accumulate, fatigue follows quickly.
Practically: LT corresponds roughly to "comfortably hard" — you can speak in short sentences but not paragraphs. Most recreational athletes train too hard on easy days and not hard enough on hard days, clustering around a "moderately uncomfortable" zone that stimulates neither adaptation.
Related: progressive-overload, periodization-notes