It's a macro. It takes the result of the prior operation and inserts it as the first parameter of the next operation. More or less it rewrites String.down

It's a macro. It takes the result of the prior operation and inserts it as the first parameter of the next operation. More or less it rewrites String.downcase("FOO\0BAR") |> String.split("\0") to String.split(String.downcase("FOO\0BAR"), "\0")