Guitar Hero is a game played on video consoles, like Wii or Xbox. You have a plastic "guitar" with 5 colored buttons on the neck, and a single "strum" switch on the body. You play notes by holding down the correct button (or set of buttons for a "chord") with your left hand, and strumming the strum switch with your right hand.
On the screen, the "notes" to play stream down the screen, you have to hit the right notes at the right time, by pressing the buttons of the specified colors and strumming. The notes correspond, roughly, to notes of the song playing in the background, which is likely to be a classic rock song from the 70s or 80s. The notes are typically very simplified as compared to the real song, they just mimic the timing of the song.
In recent versions, you can form a whole band, with a couple guitars, a drum kit, and a microphone, and everybody has to do their thing at all the right times.
Hard to explain, sounds boring in text, but it's obvious to learn, and a whole lot of fun.
It has nothing to do with playing real guitar, it's just a fun music-themed video game. Another one of those things where 12-year-olds make you look bad by doing things that you have previously demonstrated to be impossible for humans to do. When you encounter the 12-year-old who has invested thousands of hours into practicing the game, and has achieved an impresario level of skill, you will be tempted to say "you should just learn to play a real guitar", and he will roll his eyes at your amazing level of dorky totally-not-getting-it'ness.
-harry
