Fog Overrides is a event-based fog modification system for Minecraft Forge. Fog overrides is fully client-only, installing it on the server has no effect.
The mod hooks into the Forge events that run when the fog is rendered and edits the values. This makes the mod compatible with most mods that don't hook into the fog events the same way with higher priority.
The mod is configured in the config file located in the config folder in your Minecraft directory, or through the mods menu if Cloth Config1) is installed.
If you want more information about how the config works, then you can check this page as well: config