See the Wikipedia page on Perlin noise for more information.
Perlin noise is an Expression that takes in two values, an X and Y position, and returns a single value between 0 and 1. The value returned by Perlin noise is a psuedo-random value and can form pleasingly random yet uniformly distributed patterns when viewed alongside neighboring values.
What do I use this for?
This is useful when you want something to feel random, but to change smoothly through space. For example, you could generate random terrain by using the X and Y of the card space as inputs, and assigning the Perlin output to the height of the hills at that location.