Can someone explain what this is? I can't find anything on the website that explains it
As far as I know, it at least partially fulfills the role of "enterprise application integration" pattern; the idea is sort of proto-servicemesh where you have bunch of weird enterprise applications all around and you need to get them talking to each other, so you plop in a fancy EAI middleware thingy in the middle which then will talk to each of the applications individually, converting and transforming data from one format and protocol to another. But its not just a dumb hub, in addition to doing arbitrarily complex transformations, it can also make "routing" decisions based on the data itself. And this pattern being product of its time, instead of defining the network in code in some nice DSL everything is just "configured" and lots of things is achieved through clicketyclicking through GUI. I suspect this is at least partially due "configuring a turn-key product" sounding less scary to managers than "developing a system on framework".
If you look at the docs page of NiFi[1] and scroll quickly through the long list in the left sidebar with all the "processors", you already can get an idea what sort of things it does.
While this sort of solutions can be very useful in some situations, I wouldn't necessarily start designing a greenfield architecture around NiFi. But if you end up running it, then you might find that piece by piece it will accumulate all sorts of bits and bops of things because it's "convenient" to throw it in there, while the logic flows become more and more eldritch in nature.