explaining Monadic for comprehension about scala-with-cats HOT 1 OPEN

def stringDivideBy(aStr: String, bStr: String): Option[Int] = for {
    aNum <- parseInt(aStr)
    bNum <- parseInt(bStr)
    ans  <- divide(aNum, bNum)
} yield ans

parseInt(aStr).flatMap { aNum =>
        parseInt(bStr).flatMap { bNum =>
          divide(aNum, bNum)
        }
      }