When you engage in high intensity exercise, the glucose in your body gets converted to lactic acid under high oxygen demand. The lactic acid is converted to fuel in muscle cells and provides the working muscle with energy. Lactate accumulation occurs when your body stores more lactic acid than it can burn and use as energy. A buildup of lactic acid can make muscles feel sore or tired.
With moderate exercise done at a steady pace, lactate is absorbed and cleared out quickly. With high-intensity training, more lactate is produced than absorbed, causing fatigue and reducing the power of muscle contractions.
You can learn more about how lactate accumulation can impact your training and performance here.