"meteor-rejects-addon-0.3--3-.jar" appears to be a Java archive (JAR) named like a software addon or plugin, likely for a project or system called "Meteor" (or a similarly named platform). The filename suggests:
Copy and paste the downloaded meteor-rejects-addon-0.3--3-.jar directly into this folder alongside your base Meteor Client jar. meteor-rejects-addon-0.3--3-.jar