I've not done this lesson, so I'm confused about what you're trying to achieve with the line:
product = product * ????;
However, I might be able to help clarify the argument passed into "multiplyAll" which may in turn help you understand that product line.
So, "arr" in this instance is a Multidimensional Array, or Array of Arrays. The first for loop says, "Okay, we have an array with 3 elements, lets loop through those." At this point, it doesn't know or care that each element in the array is in fact another array. Now the second for loop grabs the element retrieved by the first for loop and says, "Okay, I know you're an array, lets loop through each element."
So what I think you want to happen here is put that product assignment line where your console log is currently.
product *= arr[i][j];