We're thinking about the region beyond the termination shock. And the termination shock is a region where the supersonic solar wind go through a shock You can think about a big waterfall that the solar wind goes and starts slowing down as it's approaching the interstellar medium. Now the heliopause is what separates us from the rest of the galaxy, the interstellar medium. So the Sun with its wind, it's curved like a bubble in the interstellar medium and the wind coming from the sun clashes against the interstellar medium and forms this boundary that we call the heliopause.