Fog season is a time of year when the air outside looks really cloudy and misty. This happens because the air is holding a lot of tiny water droplets that are too small to see by themselves. When a lot of these droplets come close together, they create a big cloud that you can see.
In some parts of the world, like in the San Francisco Bay area, there is a fog season that happens in the late summer and early fall. This is because there is warm air over the land and cooler air over the ocean, and when they mix together, the moisture in the air turns into fog.
Fog can be really fun to play in, but it can also be dangerous for drivers because it makes it hard to see the road. So it's important to be careful when there's a lot of fog outside.