Decoding was the process of converting information such as code into a recognizable form.
In 2266, the cryptography section of the USS Enterprise was decoding a Romulan transmission from a Bird-of-Prey when Lieutenant Stiles "suggested" giving it to the Vulcan Spock to possibly translate, due to the resemblance between Vulcans and Romulans. Captain James T. Kirk assumed that Stiles complimented Spock on his ability to decode, but Stiles wasn't sure. Kirk then stated to leave his bigotry in his quarters. (TOS: "Balance of Terror")
When the Enterprise received a seemingly gibberish communication in 2267 from a source later determined to be Nomad, Lieutenant Uhura informed Captain Kirk that it was "impossible to get any clearer" from her station, Kirk had Uhura send the message to the analysis sector to have them study and decode it. In relaying her message, Uhura stated, "Analysis sector, channel 1, main computer, decode." (TOS: "The Changeling")