Lactic acid fermentation happens during an intensive workout in your muscles. Even though you breathe heavily, you can't get enough oxygen to your muscle cells--thus, they can't carry out cellular respiration and they have to do fermentation. Lactic acid fermentation is when pyruvic acid in your cell (which was previously obtained from glycolysis) converts itself into lactic acid, which in some complicated chemical way makes NADH, which makes NAD+, WHICH goes back to glycolysis to make more pyruvic acid to make more lactic acid to make more NADH to make more NAD+ to go back to glycolysis, etc... this is what your muscle cells do to get ATP when they aren't getting enough oxygen to carry out cellular respiration, since C.R. makes 38 ATP and glycolysis makes 2. To stay alive.
ALSO - lactic acid is the stuff that makes your muscles hurt during an intensive workout.
(I have a test on this stuff in 2 days)