Red light means...

At tugboat "Holland"...
In this case the red light means that the main engine is on turning gear so it is dangerous to start it.
We turn the engine regularly, that's better for the bearings.
We turn it by means of en electric motor, with a gearwheel that meshes in teeth at the flywheel.
Not an easy operation, we need crowbars and sledgehammers to get it into place.