Admin shares usually have a name with a $ at the end. These can include 'C$' for example, which gives you remote access to the complete C: drive. FileBrowser shows these as "C:" instead of "C$".
You can also create your own Admin shares in Windows by giving them a name with a $ at the end when you create the share.
From FileBrowser's HomeScreen
Tap the 'Config' icon bottom left.
Tap 'Admin Settings'.
Scroll down to the Settings section and tap 'Show Files'.
Switch on 'Show Admin Shares'.
Admin shares are not available on Windows 10 Home.
The account you used to connect with does not have Administrator privileges.