pn is perlin noise. It's not part of the k standard, but provided by the iKe environment here. iKe is also providing 'f' which is the frame counter, and pi which is obviously pi.

It's a bit confused by the first function. It takes two arguments, x and y, and returns a tuple. The first value is 'y' and the second value is (sin(pi % 100 * y) * -18) * perlinNoise(frameNumber % 20, 0.3 * x, ), 0.1 * y). I might be a little off on that formula, it's been awhile since I've read k code.