To allow multiple user sessions per user, navigate to System Settings (1) > Security tab (2) > Allow multiple sessions (3):

Select 'Yes' to allow users to have multiple sessions. 

You can also enable this feature for specific user types (4), allowing you to specify which user types can have multiple sessions.

Having this functionality enabled allows the user to log into their account on different devices or browsers concurrently.

When disabled, a user who is already logged into one device will automatically be logged out if they successfully log in from a different browser or device.