What does “Monthly Shares” mean?

It means the number of studies/patients per month that are shared using Share feature to guest who do not have an account.

Share feature allows you to share a study or patient to an Institution/Role/Account or guest.
If you share a study, they can view that study.
If you share a patient, they can view all the studies of that patient.

If you share a study/patient to an Institution/Role/Account, they can view the studies of the shared study/patient by logging in or accessing the URL.
If you share a study/patient to a guest, they can view the shared study/patient by accessing the URL with an access key.

Monthly Shares will be counted only if you share to a guest.