As a boat cruises through the ocean, water at the bow of the vessel will be pushed against water flowing in the opposite direction. The resulting wave is descriptively known as a "bow wave," which trails away from the boat as it continues on its way. Now imagine a star, powering through the interstellar medium. That star is itself pumping out stellar gases. In the direction the star is travelling, the stellar winds buffer against the interstellar gases and a huge bow shock.