Every ellipse has two foci. The foci are, Khan Academy tells us, "two points whose sum of distances from any point on the ellipse is always the same."
Let's say that you build a miniature golf course shaped like an ellipse and place the hole at one focus. If you hit the ball from the other focus with a bank shot, the ball will land in the hole. If you hit the ball from any other point with a bank shot, the ball will miss the hole.
That's the premise of this video by YouTuber Constructive Chaos. He build increasingly complex combinations of ellipses to make mathematically predictable miniature golf course.
-via The Awesomer
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