What does adding a subscription to a member do exactly?

I'm having to move some people off of a different membership plugin and I get what adding someone to a member level does, and I get how a subscription works during the sign up process - but what exactly does it do when you are in the edit users panel? Does it send an invoice? Does it set up a payment? If I'm using PayPal express? Can I have members only have levels and not subscriptions?

Thanks again for all the great fast support everyone here has offered. I really appreciate it.