In Forge, "fluids" is a catch-all term representing both liquids and gasses in Minecraft. You can register your own fluids, define their appearance and functionality, and interact with them in different ways.
Before being used, fluids first need to be registered with the Forge FluidRegistry. This acts as a list of all fluids that Forge knows about, as well as their properties and appearance.
To get started, you'll want to first create a class to manage your fluids. A common name is ModFluids. It serves the same purpose as ModBlocks or ModItems classes.