Lactate or lactic acid is a by-product of energy production as a result of intensive exertion.
If lactic acid accumulates, then it is always an indication that the muscles do not have sufficient amounts of oxygen for release of energy and so instead the body breaks down glucose.
Starting Strength, 3rd edition,
What do the values mean?
The lactate concentration of resting blood levels are about one to two mmol/l. A workload which causes the lactate concentration to be lower than 2 mmol/l is categorised as aerobic.
The aerobic-anaerobic threshold is considered to be a lactate concentration between two and four mmol/l. A lactate concentration above four mmol/l is categorised as anaerobic.
You Are Your Own Gym: The Bible of Bodyweight Exercises, Mark Lauren, Joshua Clark