A reactionless drive is basically a closed box with the ability to just start moving, on its own, without touching or exuding anything. The classic sci-fi tropes of silent flying cars or hovering UFOs are examples of reactionless drives. The problem with reactionless drives, fundamentally, is that Newton’s third law has no (known) exceptions. There are plenty of ideas floating around about how to create reactionless drives, such as the Woodward Effect or the Albecurrie warp drive. If someone ever manages to create a self-moving, reactionless drive, then all the theories about why that’s impossible go out the window.