I can't figure out what the icon between my settings and lock-screen icon in Gnome3 is supposed to do.
It has two states, locked and unlocked, but it's not the lock screen. When I click it, it toggles its status.
That button locks/unlocks screen orientation on devices that can detect which way they are held - usually touchscreen devices - see Screen orientation section on the gnome wiki touchscreen page: Easy ability to temporarily disable and enable auto-rotation (rotation lock).
It was introduced a couple of years ago: Add an orientation lock action button. The icon used is
rotation-locked-symbolic and it's counterpart is
rotation-allowed-symbolic. They're under
/usr/share/icons/Adwaita/scalable/status and they're both part of the symbolic icons set.
On some systems with hard disk accelerometers that button appears even if your device doesn't support screen orientation detection. If that's the case you can disable the gnome-settings-daemon plugin via
gsettings set org.gnome.settings-daemon.plugins.orientation active false