What is the heliopause?

The heliopause is the boundary between solar wind and interstellar wind, where there is a balance like the outside of a bubble. It is hard to imagine there being "wind" so far in space but the solar wind is gas that is electrically charged from the sun. It travels at high speeds until it reaches the terminal shock which slows down the "wind". This is the Heliosheath. The heliopause is in the middle where the winds are balanced and then the Bow shock is where the solar wind is bent back like a bow due to the interstellar wind.

Interestingly, Voyager 2 has recently passed the heliopause on 5 November 2018.

https://www.nasa.gov/image-article/heliosphere-4/

https://en.wikipedia.org/wiki/Heliosphere

https://en.wikipedia.org/wiki/Voyager_2