What is pilot plant fermenters?

It's self-evident if you understand the basic terminology.

A "pilot plant" is sort of like a smaller-scale mockup of a factory. It's used to prove that a particular design will work and produce the desired substance, without going to the expense of building a full-scale production facility.

A "fermenter" is, basically, a big tank in which bacteria or fungi "ferment" some feedstock and produce the desired substance. What this substance is varies.

A pilot plant fermenter, then, would be a small ("small" is relative here, it could be several hundred gallons) tank in which fermentation of some kind takes place.