Well, it's a wide topic, lots of things to explain... actually, the challenge you share has some good explanation on the topic...
Basically you have an image that contains small pieces that are used like bricks to build a level with them. Every piece (brick) of the image gets a number assigned and you can draw a full level just definning those numbers as a 2D matrix. When drawing the level, every number is checked and the corresponding piece of the image is drawn.
The example you link doesn't use raylib strictly, it actually uses only rlgl, a lower level library used internally by raylib.
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Basically you have an image that contains small pieces that are used like bricks to build a level with them. Every piece (brick) of the image gets a number assigned and you can draw a full level just definning those numbers as a 2D matrix. When drawing the level, every number is checked and the corresponding piece of the image is drawn.
The example you link doesn't use raylib strictly, it actually uses only rlgl, a lower level library used internally by raylib.