It will give you ['even', 'odd'][0] which means 'even' (the value of the first index of the array ) or ['even', 'odd'][1] which means 'odd' (the second value of the second index of the array).
Similar to myArray = ['a', 'b', 'c'], where myArray[0] = 'a', myArray[1] = 'b', myArray[2] = 'c'.

It's an array, the 0 and 1 are the indexes of this array, friend.

by [...]

Ok, I understand that but how is "even" or "odd" determined based on the reducer returning 0 or 1?
I understand that 0 is the first element of the even,odd array and 1 is the second element, but how is the value passed in? I apologize if my phrasing is poor, I'm trying to learn ES6 right now.

It will give you either 0 or 1.

How exactly does this work ? I understand the reducer section, but how does this arrow function interact with ["even, odd"]?