Glow Plug
A glow plug is a small heating element in each cylinder that preheats the combustion chamber before starting. Diesels ignite fuel with compression heat, and in cold weather they need help.
When you turn the key and see the coil symbol on the dash, that is the glow plugs warming up. Wait for the light to go out before cranking in winter.
Failed glow plugs cause hard starting and white smoke on cold mornings. They are tested with a simple resistance check and are usually cheap to replace.