By default, your QGIS installation includes a single user profile called default. But you can create as many user profiles as you want.
If you have more than one profile in your QGIS installation, the name of the active profile is displayed in the application title bar in square brackets.
Because each user profile contains isolated settings, plugins, and history, they can be great for different workflows, demonstrations, users on the same computer or testing settings, etc.
You can switch between them by selecting them in Settings.
Unless changed, the profile of the last closed QGIS session will be used in subsequent QGIS sessions.