Lactic acid is a result of anaerobic muscle contraction. Your body is constantly making lactic acid, but you can only feel it at high intensity anaerobic work because it is not gotten rid of as fast as it's made. It is lactic acid that gives you the burn in hard workouts. Lactic threshold or LT (Also Anaerobic Threshold) is therefore a critical part of cycling workouts. Once you can raise your lactic threshold, (Your LT is a certain heart rate) you can go longer and harder without the punishing burn. This is possibly the most talked about thing on this board, so do a search. (I know next to nothing compared to some people here.)