Perlin noise is an algorithm for generating gradients in 1, 2 or 3 dimensions, largely by overlaying ‘octaves’ of noise over itself at varying frequencies.  The end result is that we can use this for creating seemingly random but realistic looking textures and fractal landscapes very easily.

The first video is a generic 3 octave Perlin noise gradient.

We can easily turn this into a 3d model, the following shows how this works by using frequency cutoffs to create terrain lines.