LT2, or Lactate Threshold 2, is the second phase of exercise during which the body produces lactate more quickly than it eliminates it, leading to fatigue. It is also referred to as the anaerobic threshold and is associated with the maximal intensity that can be sustained for roughly 30 to 60 minutes.
Your body is able to tolerate lactate when you are training at or around LT2 so you are able to continue pushing hard during races, time trials, and climbing. This is typically accomplished through training in Zone 4. Sportsmen also utilize LT2 to gauge racing and pacing through triathlons, cycling, and long-distance running.
