to discover the meaning of a message written in a code (=a set of secret signs or letters) [= decipher]
if a computer decodes data, it receives it and changes it into a form that can be used by the computer or understood by a person [≠ encode]:
The software decodes the information embedded in the satellite broadcasts.
to receive electronic or digital signals and change them into a picture and sound for a television or radio [≠ encode]:
decoding boxes for your TV set
to understand the meaning of a word rather than use a word to express meaning [≠ encode]