What does this output item in the result of the call with blocktrail api mean? This is the result of a call with blocktrail api. It is about a transaction with the hash 6b1c57d98a557b5f578c02f6c386ef049df5890ebddd01d286bf345f47c5cecc

{"raw":"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","hash":"6b1c57d98a557b5f578c02f6c386ef049df5890ebddd01d286bf345f47c5cecc","first_seen_at":"2016-05-11T14:13:03+0000","last_seen_at":"2016-05-11T14:13:03+0000","block_height":411307,"block_time":"2016-05-11T14:15:43+0000","block_hash":"0000000000000000054ba46707cc4c736f47439040992cf3edc484fbb1e5b180","confirmations":44132,"is_coinbase":false,"estimated_value":665970,"total_input_value":681400,"total_output_value":671400,"total_fee":10000,"estimated_change":5430,"estimated_change_address":"18KGUfU2BXUm1e1KtCCrmz1Mdum4rGNvWt","high_priority":false,"enough_fee":true,"contains_dust":false,"inputs":[{"index":0,"output_hash":"d688982a2e84bc5dae3ea7372e8cd74c7fa3c48653a2bd7d81b54cf4d32dfdc4","output_index":2,"value":681400,"sequence":4294967295,"address":null,"type":"multisig","multisig":"1of3","multisig_addresses":["1HhfcdD1hRaim17m5qLEwGgHY7PBTb1Dof","1FkQMTyqzD2BK5PsmWX13AeJAHz5NEw7gq","17VLRV4y7g15KNhCepYvgigHHvREzbEmRn"],"script_signature":"00473044022077396917627358151b3a95abde6407d617fc2bcf2f2056e9a9096a7fea896b4002205c11958bc0dc6de05ee0c9b1ceaef7635cafcde6653c21a1a8a8f37af5e92eee01"}],"outputs":[{"index":0,"value":5430,"address":"18KGUfU2BXUm1e1KtCCrmz1Mdum4rGNvWt","type":"pubkeyhash","multisig":null,"multisig_addresses":null,"script":"OP_DUP OP_HASH160 503d498cfa360ab247c08a9cf73dc5f7f1a0a4ee OP_EQUALVERIFY OP_CHECKSIG","script_hex":"76a914503d498cfa360ab247c08a9cf73dc5f7f1a0a4ee88ac","spent_hash":"e3a462396931aaba740a459e79058f9ed9d98a39937b1e1b5832119b6d9dab0c","spent_index":0},{"index":1,"value":0,"address":null,"type":"op_return","multisig":null,"multisig_addresses":null,"script":"OP_RETURN 04fdfb5609aecb84f9f8f11f87bb1b7f9049805d3c6bee01d2199e99","script_hex":"6a1c04fdfb5609aecb84f9f8f11f87bb1b7f9049805d3c6bee01d2199e99","spent_hash":null,"spent_index":0},{"index":2,"value":665970,"address":null,"type":"multisig","multisig":"1of3","multisig_addresses":["1HhfcdD1hRaim17m5qLEwGgHY7PBTb1Dof","1FkQMTyqzD2BK5PsmWX13AeJAHz5NEw7gq","17VLRV4y7g15KNhCepYvgigHHvREzbEmRn"],"script":"OP_1 02ae22e5d5b7efdcfac8c955c57d0c18fa40e196c023f1756a89d2d4cc01706188 031466e0f4b7ae4e5ddfe4608ba2955f3aa045f92a2d9ec036d39395e752d307a1 03e5c24ea5185f7c4165a20db9b785db320dd73637f5a05b0eeed52cb6eaccb12b OP_3 OP_CHECKMULTISIG","script_hex":"512102ae22e5d5b7efdcfac8c955c57d0c18fa40e196c023f1756a89d2d4cc0170618821031466e0f4b7ae4e5ddfe4608ba2955f3aa045f92a2d9ec036d39395e752d307a12103e5c24ea5185f7c4165a20db9b785db320dd73637f5a05b0eeed52cb6eaccb12b53ae","spent_hash":"f2fc9d4aa6c7b9de1312e80f4df744f637453b947fbf428da5dce8b3b997c741","spent_index":0}],"opt_in_rbf":false,"lock_time_timestamp":null,"lock_time_block_height":null,"size":311,"is_double_spend":false,"double_spend_in":[]}


In the "outputs" array, there is this item:

{"index":1,"value":0,"address":null,"type":"op_return","multisig":null,"multisig_addresses":null,"script":"OP_RETURN 04fdfb5609aecb84f9f8f11f87bb1b7f9049805d3c6bee01d2199e99","script_hex":"6a1c04fdfb5609aecb84f9f8f11f87bb1b7f9049805d3c6bee01d2199e99","spent_hash":null,"spent_index":0}


Why does it have the value of null for both fields "address" and "multisig_addresses"? What does "OP_RETURN" in the field "script" 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.