Fog is a hazy, cloudy layer of water vapor very that hovers close to the ground. Driving in fog feels a bit like being inside a cloud.
Fog is just what it appears to be — a cloud lying low in the atmosphere, getting its moisture from a body of water like the ocean or a river. When fog is thick, it can be dangerous to drive in, and most cars have special "fog lights" to help drivers see the road. When you're deeply confused, you can call it a fog too, and when things are unclear (literally or figuratively), they're said to fog up."