Blood Lactate or simply Lactate is a by-product of carbohydrate metabolism.
When carbohydrate is metabolized, lactic acid is produced in the form of lactate ions and hydrogen ions. The greater the rate that carbohydrate is metabolised for energy, the greater the rate that lactate is produced.
When the rate of production exceeds the rate that it can be cleared (metabolised for further energy production and buffered by bicarbonate), lactate and hydrogen ions will accumulate in the blood and muscles.
The accumulation of hydrogen ions lowers the pH of the blood and muscle (making it more acidic), which inhibits the functioning of the muscle, causing you to not be able to work as hard. Hence lactate is associated with fatigue, without in itself causing fatigue.
Therefore the concentration of lactate in the blood gives an indication of the type of energy metabolism and also training status.