This is a short, focused period. It's often 1-2 weeks long. Its purpose is to get a specific set of marketing tasks done.
Marketing is a marathon, but "sprinting" helps you build momentum. When you try to "do everything," you end up doing nothing well. A sprint forces you to pick one project—like updating your homepage or fixing your email sequence—and finish it. It prevents the "open loop" stress that leads to founder burnout.