Each Membership Level is assigned a WordPress Role.
The WordPress Role for each Membership Level can be set in the Setup > Levels *Click on Level Name* > Additional Settings section of WishList Member.
The WordPress Role for all of the Members within a Membership Level can be set accordingly. In most cases this will be set to Subscriber.
The WordPress site touches on the different types of Roles here.
Each new member will be assigned this specified WordPress Role when they join the Membership Level.
Note: If an existing Member with a different WordPress Role joins the Membership Level, they will retain their original WordPress Role.
A Member can be a part of multiple Membership Levels but they can only have one WordPress Role at a time.
If an existing Member joins or is placed into another Membership Level that has a different WordPress Role than their previous Membership Level, the Member will retain their original WordPress Role unless their account is manually updated.