From The Washington Post:
Once they get the ball, pitchers have 15 seconds to go into their windup (20 when there are base runners). If not, the ump calls a ball. Hitters have to be in the box and looking at the pitcher with eight seconds left on the pitch clock. If they’re not, the ump calls a strike.
Why It Matters
Baseball games have turned into long slogs. While good strategy, it has made it much less exciting to watch. The pitch clock will keep games moving and hopefully make baseball more exciting for viewers both in-person and at home.