A QGIS user profile is a standardized configuration of the application.
By default, a QGIS installation includes a single user profile named default. However, 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 square brackets on the application title bar.
Since each user profile contains isolated settings, plugins, and history, profiles can be very useful for different workflows, demonstrations, multiple users on the same computer, testing setups, and more.
You can switch between them by selecting the desired profile in the Settings.
Unless changed, the profile from the last closed QGIS session will be used in subsequent QGIS sessions.