It's an alias for `run`.  The term `run` comes from the Haskell API:

https://john-millikin.com/software/enumerator/reference/enumerator/latest/Data.Enumerator/#v:run

And it makes sense in Haskell, because due to the side effect free nature of Haskell, it's not until you call `run` that any other operation that you have done up to that point can be useful... this is the method that actually causes it to run (assuming you do something with the monad it returns).  In Play, calling `Enumerator.apply(iteratee)` actually causes the enumerator to start running, which makes `Enumerator.run` is a weird name, since both methods cause it to run.  So I actually find `|>>>` more readable, since I use the iteratee API all the time, I know that this means run and feed EOF, which is less confusing than just `run`.