I'm currently reading a paper that says the following about nodes entering and leaving a system: "Joins and voluntary departures are modeled by a Poisson process with a mean arrival rate of R. A rate R = 0.05 corresponds to a node joining and leaving the system every 20 seconds on average".

I've searched for poisson distribution on the internet and read a bit about the M|M|1 queue etc but I still don;t understand this. How do they get this rate R and how do they determine that it means one node every seonds or so (is it something got to do with the number of nodes overall in the system and the length of the simulation time)?

Many thanks in advance