In the "Formula" input of the Reduce node, you link it to a series of nodes that iterates over each item in the source array, just like with Filter and Map. In the Accumulator you give it the starting value eg 0. On each iteration it gives you the "result" as an option to refer to as the accumulated value to that point. So if you're adding up a value in an array, you'd have an Add node with the "result" as one number and the value from the array that you're totalling up as the other. A bit like saying result = (result + ThisValueInTheArray) as you would in code. I'll pop a screenshot of an example in a sec.
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