A lucky block is this Mario Like question mark block. But insted of growing Mushrooms or Powerups, you can get anywhere from a Pig Stack with a villager on top, so a giant hole with Lava and cobwebs (very slowww death). Basically, it's all random. Each Luckyblock addon/mod has thier own unique drops and items that have been added to the game to make things more SPONTANEOUS!
What is a lucky block?