IF Statment Logic Meaning

const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8];

const validateCred = (array) => {
  let total = 0;
  for (var i = array.length - 1; i >= 0; i--) {
    // console.log(array[i]) 
    if ((array.length - 1 - i) % 2 === 1) { /*I would like to know what does this do when put in a if statment array.length - 1 - i  */
      array[i] *= 2;
      // console.log(array[i]) 
      if (array[i] > 9) {
        array[i] -= 9;
      }
    }
    total += array[i]
    console.log(total)
  }
  return total % 10 === 0;

};
console.log(validateCred(valid1))