Have you ever lifted a heavy weight or done jump squats and felt that deep burn? When you do a strenuous activity that pushes you out of your comfort zone, your body struggles to deliver oxygen to your straining muscles as quickly as it is needed. When this happens, lactic acid is produced. Lactic acid is produced by cells that have an insufficient supply of oxygen. When this happens muscles generate energy anaerobically (without oxygen).
Breaking down glucose from food we eat is one way in which our bodies produce energy. Glucose in your body is broken down or metabolized into pyruvate through a process known as glycolysis. When the body is not straining pyruvate is shuttled around the body aerobically to be broken down for energy. When you are straining and oxygen is limited, the body converts pyruvate into lactate which allows glucose to breakdown and energy to be produced for your bodies use. When this happens your muscles will feel that deep burn.
A common misperception is that lactic acid causes muscle soreness - this is false. Although I have found that if I work out at a level where I feel that lactic burn, I can expect some soreness in the coming days, muscle soreness is actually caused by micro tears in the muscles following trauma. Lactic acid is typically out of your system an hour after an intense workout and you can minimize feeling lactic acid by lowering the intensity of your workout, or training harder which will cause your muscles to adapt, and more efficiently absorb the lactic acid.