Here's why:

* Fermentation is a metabolic process that occurs when oxygen is limited. This is different from aerobic respiration, which requires oxygen.

* Lactic acid fermentation is a specific type of fermentation that occurs in human muscle cells when they are working hard and not getting enough oxygen.

* Pyruvate, a product of glycolysis, is converted to lactate in this process.

* This process is important because it allows cells to continue producing ATP (energy) even in the absence of oxygen.

It's important to note that while fermentation produces lactic acid, it doesn't mean that this acid is necessarily the sole product. There might be other byproducts depending on the specific type of fermentation.