How does the double[].Random extension work in C#?

I'm trying to make sense of some open-source code, and I've come across this line:

int r = distribution.Random(random.NextDouble());

distribution is an array of doubles.

I've never seen this Random extension applied to an array, and I can't find references to it online. Does anyone have an idea of how it works, exactly?

(For context, this is a part of the Wave Function Collapse algorithm and that specific bit of the code aims, I believe, at drawing an index in an array depending on the probability assigned to that index.)