Just in case it may be helpful, I'll add a little more.
Everything outside your home directory is off-limits to prevent accidental damage, but at least you can look at it. In the case of /root, for some reason it's not usual to set any access: if you select it in the file-manager, right click, and check Permissions, you'll get the point.
If you open a terminal, you can make yourself root user with
and then access it from the command line, directly or by running nautilus.