Like it's solo cousin, air guitar, an air band involves pretending to play musical instruments enthusiastically to recorded music. No actual instruments are involved.
Hey John, would you like to join my air band? We still need an air drummer and an air singer.
When you're listening to a really good song and can't decide what instrument to pretend to play, so you switch up every few seconds. Most common instruments include: lead guitar, rhythm guitar, drums, and keyboard.
I can't listen to Spraynard cuz I always start air banding really hard.
"I can't decide between picking the guitar, strumming the guitar, or drumming, so I'm just gonna air band."
"Dude, who CAN decide?"