Perlin Noise is essentially an algorithm to generate lots of random numbers which are close together - to form a gradient of sorts. You’ve probably seen a gradient before, maybe in a Powerpoint slide or as the background of a webpage; it’s basically something that transitions between two colours.
We call this gradient ‘noise’, and it can affect our data in ways which seem organic. The numbers by themselves might look a little meaningless and useless, but we can apply these numbers to do many interesting things, such as procedurally creating patterned textures for materials like marble, or creating clouds, or for creating entire landscapes!
This website will walk you through the process of creating this Perlin Noise to use yourself for your own projects, with the primary example of creating the heightmap for some terrain.