Understanding &Enum.reduce/3

gistfile1.ex

iex(11)> Enum.reduce([1, 2, 3, 4, 5], 0, fn(x, acc) -> IO.puts "x: #{x} acc: #{acc}";x + acc end)
x: 1 acc: 0
x: 2 acc: 1
x: 3 acc: 3
x: 4 acc: 6
x: 5 acc: 10
15
iex(12)>