On your Contacts page, you'll see each contact listed as either a customer or a lead. Here's what each of those means:
Customer
A customer is anyone who has a subscription associated with them.
Lead
A lead is anyone else who has been added to the system without a subscription associated with them; they can be created by:
A member of your team manually from the "Add a Contact" form.
They placed a booking via your Storefront but didn't set up their payment.
They completed a form on your Storefront
Any lead who has come in from the Storefront will also be listed as a Prospect.
Prospects is a dedicated area for managing and actioning your Storefront leads.