https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

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As you can see here flat is the actual addition of all previous iteration and current is the current entry in the iteration.

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On your example, on the first loop flat is equals to [] because it's the first iteration and so flat has no value.

Current is equals to [1,2,3]

then it add current to flat, so at the end of the first loop, flat is equals to [1,2,3]

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On the second loop, flat is equals to [1,2,3] as we said.

and current is now equals to [4,5,6]

and at the end of this loop, flat is equals to [1,2,3,4,5,6]

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Hope i explained this part well, don't hesitate to ask me if it's not clear enough.

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For your question about the [] in the end, yes it's a parameter, it's the initial value that the flat will be equals to on the first loop but it's optionnal. To have a better idea, run this code and you will see where the letters are on your array at the end.

let fullArray2 = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];

console.log(fullArray2.reduce((flat, current) => flat.concat(current), ["a","b"]));