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.