Minecraft's perlin noise generator draws multiple layers, called octaves, which have different strength values (amplitudes). Lower levels of detail make the large, general shapes like the positions of hills and rivers. They have the biggest impact, but only make broad strokes. Finer levels of detail build on that, making it rougher and more natural. In math terms, random numbers from each pass of the perlin noise generator change twice as fast as the last one, pass 2 is twice the detail and pass 3 is 4x the detail. This random number is then multiplied by that element of the amplitudes list. firstOctave allows you to skip the first <x> layers, making the first 2x, 4x, 8x, ... as detailed.



Each element of Amplitudes after the first adds a new double-detail perlin noise layer, and the value determines how much of an impact it has on the final result. Generally you want the first element to have the highest value to determine the layout of the area and make the others much lower to add detail to it.