From here https://www.paradoxinteractive.com/games/cities-skylines-ii/features/public-cargo-transportation : "When creating a bus line you can set waypoints that control the route taken by the bus from stop to stop, allowing it to avoid busy roads or intersections."

So I think he's talking about this. You can force buses to take a certain route without having to place a stop, this can be useful if the game calculates the default route through some busy area but you know a better route like a dedicated bus street or something.