Move Lock is a bridge-based feature that does exactly what its name says, it locks your avatar into a fixed position, preventing your avatar from being pushed or moved by other avatars or objects.
Using Move Lock
To turn Move Lock on or off:
Go to the Avatar menu
→ Movement sub menu and click Move Lock
You will see “Movelock enabled” or “Movelock disabled” in Nearby Chat, and you will hear a sound
If Move Lock does not enable or disable, recreate your bridge: Avatar menu → Avatar Health → Recreate LSL Bridge
You can control the Move Lock behavior in Firestorm 4.6.5 and newer via Preferences → Move & View → Movement:
Always re-lock position after region changes > When enabled, Move Lock is disabled during teleport, then re-enabled it after you arrive. When disabled, Move Lock is not re-enabled after a teleport
Lock and unlock position after stopping or starting movement > When enabled, Move Lock is temporarily disabled when you manually move your avatar, and re-enabled when you stop moving. When disabled, you will be unable to move your avatar while Move Lock is enabled.
Also in Preferences → Firestorm → Extras, at the bottom, is Enable LSL-Client Bridge. This must be enabled in order to use Move Lock.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.