The function has two arguments. This just means that there are exactly two inputs to the function.

This equation multiply(arr, n) == multiply(arr, n - 1) * arr[n - 1]
means that if you call multiply(arr, n), the result should be exactly the same as if you called multiply(arr, n-1) and multiplied the result of that function call by arr[n-1]. Which is to say, the product of the first n elements in the array is the same thing as the product of the first n-1 elements in the array multiplied by the nth element in the array.

Recursion is all about calling the function from inside itself with a reduced set of inputs until the problem is very easy to solve.

I wouldn’t skip the lesson outright. Recursion is hard in part because it highlights all of your misunderstandings and holes in your knowledge, and it is important to sort out those problems.