The line along which you can see something.
Sightline is about being able to see things clearly or not being able to see them at all.
Examples:
I can't see the stage because someone is blocking my sightline.
The sightline from the top of the mountain is very beautiful.
The sightline in the forest is blocked by the tall trees.
I need to find a good sightline to see the fireworks.
The sightline from the airplane is amazing!
The sightline from the top of the tower is the best in the whole city.