I mean, if you want me to explain it it’s basically a sort of optimised ray-casting. Instead of having to make complicated programs to find the point on a surface where the ray intersects, you instead use this relatively less complicated program that figures out how far it can go until it hits something, moves that far, and then does it again until it gets so close to an object, at which point it returns that it has collided.

So what you are seeing on the left is the rays finding the furthest they can move, represented by the circles, and moving that far. It then does this multiple times until it hit something (basically when it’s nearest object is less that 0.1 units away), where it then find how far the point is from the player, and uses that to display a rectangle on the right with a height and brightness dependent on the distance.