Automatic renewals:

If the member has added a card to the system themselves, the membership will renew automatically on their renewal date each month.

Renewal by online payment:

If the member does not have a card on the system, they will get an email asking them to make an online payment to renew their membership.

Manual Renewals:

If online payments are disabled, then you would need to collect the money from the customer yourself and manually mark the membership as paid each month.