This is the opening number to a musical review of Sondheim with a stellar (no, STELLAR!) cast (Carol Burnette, Ruthie Henshall, George Hearn, John Barrowman, and Bronson Pinchot - who sings this number). I HIGHLY recommend renting the DVD "Putting it Together", I love almost every minute of it (and highly like the rest).
This always makes me laugh though, especially if you've ever been to a live performance of anything. Basically it's about the etiquette, but extremely funny!