Indy is a skateboarding
term to describe grabbing the toe edge of the board with the trailing hand.
While the indy is a grab trick all on it's own you'll usually see "indy" as a modifier for other tricks. If you do an indy grab then bone both legs you have an Indy Stiffy. If you do a nosebone with your trailing hand, you got it, Indy Nosebone. If you tweak your legs and grab your toe edge with the rear hand: Indy Tweak.
The indy is the first trick you'll learn when you get on the half-pipe, even before the mute grab. It's easier to keep your balance with the front arm when you are learning to air and it's natural to be afraid of losing your board so you'll instictively want to hold it on your feet with your rear arm.