Fabric is one of many mod loaders ala Forge. It’s newer and less bulky then Forge (but afaik it already did have it’s own drama so now we also have a fork called Quilt, the same goes for Forge and NeoForge).
The mods I’ve specified above can be considered as a suite replacement for the (old) OptiFine.
E: For example this all the mod loaders modrinth (mod hosting website, curseforge alternative) currently lists: