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, since you created your account.
Your quota is not reset every month. Instead, Participant count is getting built-up in your account from the moment it is created.
That means that, if you have created many campaigns on Viral Loops (did you know you could create unlimited campaigns?!) and some person has joined multiple campaigns, they are counted only once.
Makes sense? Let us know if you need more help :)