What is blocking and why is it needed on stage and on set?

When on stage and on set actors need to know what to do in regards to motion. This practice of motion is called blocking. Blocking is needed because the director has to show what motions he or she wants from the actors while on stage performing. It's also important for the actors to practice these motions repetitiously so the play will run smoothly. Although mainly used in theatre blocking is used in many other entertainment aspects as well.