Hi,
It's active users in database.
This means that you can have 1.000.000 users, but only 100 can be active at the same time. So, it's like saying that you can have a maximum of 100 concurrent users.
But be aware that the personal environment is on a shared server and has very little resources. If the applications running on it start consuming too much resources, the environment may be shut down to avoid impacting other environments, I think.
I already saw that happen, at least.
Cheers