Imagine you are standing in line to buy a ticket at the counter. If you, after one minute, observe the number of customers that are behind you it might be looked upon as a (rough) estimate of the number of customers entering the system (here, waiting line) per unit time (here, minute). If you then divide the number of customers in front of you with this ”flow” of customers you just estimated the waiting time you should expect; i.e. the time it will take you to reach the counter, and indeed it is a rough estimate.