This noun refers to any agent or substance, such are bacterium, mould, yeast or enzyme which causes fermentation.
As a verb, ‘ferment’ is a process by which the original matter is broken down into its various components. As an example, food that is left out of the ‘fridge will eventually ferment because it is exposed to bacteria in the air.
Metaphorically, it is used to describe a situation in the context of a commotion or unrest.
It arises in 15 Century Latin from fermentum (meaning yeast) from fervere meaning ‘to seethe’.
Note how the physical action reflects the metaphorical meaning.