If we assume events are independent (the occurrence of one event does not affect the probability that a second event will occur), then the counts per unit interval can be assumed a random variable that follows a probability distribution. Counts, i.e., the number of times an event occurs in an interval follows a Poisson distribution.
Poisson distribution has one control parameter. It is the rate of occurrence; the average number of events per unit interval.
In lesson 36, we learn the fundamentals of Poisson distribution. You will also meet Able and Mumble two of my friends.