Why does some transaction returned in the result of Blocktrail api call contain a field called "raw"? Such as the result of https://api.blocktrail.com/v1/btc/transaction/57ecab25858a91448cbca713ea369c23a06f6080fca85dea367c33fad93fa056?api_key=MY_APIKEY

Also what exactly does "raw" mean here? Could it be translated to some information?
It looks like a hex encoding of the binary encoding that Bitcoin uses. You could also get this through the getrawtransaction Bitcoin RPC call, if you have txindex=1 enabled.