You have the plugin User Switching installed
From their FAQ:
What does “Switch off” mean?
Switching off logs you out of your account but retains your user ID in
an authentication cookie so you can switch straight back without
having to log in again manually. It’s akin to switching to no user,
and being able to switch back.The Switch Off link can be found in your profile menu in the WordPress
toolbar. Once you’ve switched off you’ll see a Switch back link in the
footer of your site.
From my experience:
The plugin allows you to switch to another user’s profile and see what that user sees without knowing their login credentials. After you’re done, you click on switch off to go back to using your profile.