This sounds like some kind of interview question, refresh metadata retrieves information about the target application like number of dimensions, type, names, what you see in dimension details
refresh members does what it says and refresh dimension members from the target application.
There is no set frequency to refreshing.
Refresh Metadata - brings all the dimension (this is one time if needed, If you delete or add a dimension)
Refresh Members - Brings in all the dimension members (as needed if new members are added)