This clarifies that Meteor is not a standalone launcher or a resource pack. It is a that requires a mod loader (Fabric) to inject its code into the Minecraft game files.
.jar (Java Archive) is the standard package format for Java programs and mods. This extension tells your operating system to execute the file using Java Runtime Environment (JRE). Do not attempt to unzip it manually; mod loaders expect the .jar intact. File name- Meteor-Client-Mod-Fabric-1.20.jar