(Maybe off topic) Can someone explain what a finite state machine is?

[Matty Sarro]

Hey everyone. I am currently reading through an RFC, and it mentions
that a client and server half of a transaction are embodied by finite
state machines. I am reading through the wikipedia article for finite
state machines, and sadly it's going a bit above my head. I don't
really have a background in engineering, and would really love to
understand what is being said. Does anyone have a simple way to
explain it?