The Lock option is an "On/Off" toggle button for locking or unlocking a layer. It is located to the right of the visibility icon and indicates whether or not a layer is locked. Press on the Layers Button in the Top Toolbar to access the Layer Lock toggle.
To lock (or unlock) a layer in the Layers Menu, press on the lock icon of the layer to be toggled. A closed lock icon indicates that the layer is locked and an open lock icon indicates a layer is unlocked.