Performers who see allegro con spirito as an instruction in a piece of music know that they need to sing or play it in a fast and lively way.
Allegro means "brisk" or "fast" in Italian, con is "with," and spirito means just what it looks like: "spirit." So allegro con spirito means "fast and with spirit" — in other words, lively in both tempo and style. There should be some bounce to it! As you can imagine, this instruction generally appears in music that expresses a happy mood.