Locked

Locked is a property on Roblox Studio that is displayed as a lock sign. It makes the piece or model unselectable, meaning if one runs his/her mouse over it is, stays on the pointer and cannot be moved. When in-game, if you try to modify a locked block with building tools it won't allow you.

To unlock, use the lock tool on the locked piece again, unlocking it and making it editable. Another way is to select the piece via the explorer, and on the properties, untick the "Locked" property.