Every visitor that joins one of your campaigns (either directly or through a referral) counts as a Participant.
Participants are counted uniquely, across all your campaigns, starting the day you created your account.
Your quota is not reset every month. Instead, the Participant count is getting built up in your account from the moment it is created.
That means that, if you have created many campaigns in Viral Loops (did you know you could create unlimited campaigns?!) and some person has joined multiple campaigns, they are only counted once.
Makes sense? Let us know if you need more help :)