csmith12 gives great information. I'll add that I know a woman who recently bought a brand new Sym Wolf 150, rode it away from the dealer, and while turning in a parking lot with the clutch lever completely released, applied a little throttle. The driveline lash, which is what you're asking about, caused a lurch big enough to make her lose her balance, and she fell and hurt herself enough to scare her into selling the motorcycle.
Most motorcycles have very noticeable driveline lash, and we get used to keeping it under control, as csmith said, by slipping the clutch a little, or by shifting to a higher gear, or by being careful with the throttle, etc.. A lot of the lash comes from the dog clutches in the transmission that have a lot of lash by design, so you can get them into engagement easily with things spinning fast. Some racing transmissions have half the dogs cut off to make engagement easier. You think you have lash with all the dogs, you should try half the normal number!
You can see one of a transmission's dog clutches in this photo. There are several types, but this is one type used in the Ninja 250. A pair of dogs is circled. You can visualize how much lash there is if you rotate a gear the other way, making the dogs separate and then rotate until they contact the next one, and this is what happens when you let off the throttle or re-apply it.
