By default, ArcGIS Pro uses the Named User license type. A Named User license is based on credentials—a user name and password—associated with a person rather than an authorization number associated with a computer or server.
For every ArcGIS Desktop license in your workplace, you can add one member, also called a named user, to an ArcGIS Online organization. The member has a user name, assigned by the ArcGIS Online administrator, and a password, chosen by the member. Once a member has been added to the ArcGIS Online organization, the administrator can assign an ArcGIS Pro license to them. The license configuration is done in the ArcGIS Online web application.
As a software user, once a license is assigned to you, you can sign in to ArcGIS Pro with your ArcGIS Online user name and password. You cannot use ArcGIS Pro without being signed in. However, once you sign in the first time, you are subsequently signed in automatically by default.
Because ArcGIS Pro Named User licenses are configured in ArcGIS Online, you must have an ArcGIS Online organization up and running ahead of time.
For an in-depth demonstration of setting up an ArcGIS Online organization and configuring a Named User license, watch this video.