foldleft is a partial applied function (curried), where first it is applied an initial value followed by an operation on a pair of elements from the sequence to be fold:
def foldLeft[B](z: B)(op: (B, A) ⇒ B): B
scala> val xs: List[Int] = List(1,2,3)
xs: List[Int] = List(1, 2, 3)
scala> xs.foldLeft(0){(acc, x) => acc + x}
res9: Int = 6
scala> xs.foldLeft(0)(_+_)
res10: Int = 6
