A bifold door sightline is the width of the frames that sit between each pane of glass.
The way bifold doors are constructed, each panel of glass is housed in its own frame. The width of those two frames together forms the sightline.
Sightlines are often vital when it comes to external bifold doors but they are important with internal bifold doors too. They will majorly influence the visibility between your rooms when the door is closed, as well as affect the visual balance and style of the door itself.