Hi there and welcome to the forums!
Essentially what this array method is there for is to simplify the combination of array elements. So lets think about the example array [1, 2, 3, 4, 5]
. If we wanted to get the sum of all elements of this array we could do this in a few ways.

let array = [1, 2, 3, 4, 5];
let total = 0;
total += array[0];
total += array[1];
total += array[2];
total += array[3];
total += array[4];
This would be the most basic way of doing it. Now of course this can be done more efficiently using a for loop like so.
2.
let array = [1, 2, 3, 4, 5];
let total = 0;
for (let i = 0; i < array.length; i++) {
total += array[i];
}
This simplifies part 1 a bit but is still a bit excessive. However you can see the general idea. We have a total variable that we add each element to, eventually ending up with the final result of the sum of all variables.
 The reduce method
let array = [1, 2, 3, 4, 5]
let total = array.reduce((total, arrayValue) => total + arrayValue);
We can see immediately that this is a lot more compact than the last two methods, fitting into only 2 lines (and even a single line if you don’t assign the array first like so.
[1, 2, 3, 4, 5].reduce((total, arrayValue) => total + arrayValue);
All of the above code blocks are doing exactly the same thing, it’s just the reduce method is a lot cleaner in the way it is written. All it’s doing is collapsing the array into a single value based on the function provided same as any of the above code blocks were doing.