“The bees’ knees” is an old-fashioned expression for something like “cool”. Example: “This cocktail is amazing!” “Oh yeah, it’s the bees’ knees!”
Barbershop is four-part close harmony singing. You may have seen a barbershop quartet before without realizing it: They appear in The Music Man, in Scrubs, on Jimmy Fallon. Barbershop music is full of seventh chords, syncopations, and play between the different voices. The four parts of barbershop singing are tenor, lead, baritone, and bass. Singing barbershop can be a challenge, but that’s what makes it fun! As Deke Sharon puts it: It’s the martial arts of a cappella singing.