A joke that's formed by two or more people. Normally it's just an excuse to laugh because those two or more people have little to no comedic value and they're probably the unfunny friend in a group who always tells jokes. Try to explain what an inside joke literally is and you'll find out you can't.
"Feathers on the bird"
"Feathers on a train xD"
"Yeah shut up"
Inside jokes are almost as annoying as little preppy girls spouting Jonas bros are sexy, I don't find any of them ever funny, I never have made one up, and people who do should be shot. (Might be extreme but it really is grueling)
"Dude you have any inside jokes?"
"Yeah, one day me and my friend were at a lake and the lake exploded for no reason, now we always laugh whenever someone says 'random'"
"Because the lake explosion was so random hahaha"
"Yeah that doesn't make any sense. If it was funny to start then fine ok, but just the general term of random isn't funny, and a lake exploding isn't funny."
"Well it's an inside joke"