What does this output item in the result of the call with blocktrail api mean?
You can find a full explanation of OPRETURN here: https://en.bitcoin.it/wiki/OP_RETURN

TLDR; OPRETURN is a "data push" output, it allows for putting up to 80 bytes of data into the transaction that have effect on the transaction.
And the value for that output can be 0.

It is used for various things, amongst others; publishing "proof of existence" hashes in the blockchain and meta protocols.