We define an "Active User" to be a user who has generated a lead during the last month (i.e billing period).
Some plans have limits on the number of "Active User" in the month.
For example, if you are on the Starter Plan then you have a limit of 10 Active Users per month.
Suppose you have Person A sends generated a lead the month before, but they then leave the company. Then this means that since this user will not generate any more leads during the coming months then they will not count as an active user from now on.