A message is a complex idea, that you may want to communicate to another party. Suppose for example that you want your friend to feed the cat. You could communicate your request with spoken words, by a note, by pantomime or even by some secret signal.
(We can imagine, for instance, some secret agents setting up a convention of a rose in a person’s lapel and an obscure book held in the right hand, or a chalk mark on a park bench to indicate that there is a stash of supplies at a certain place. That would be an unconventional message, but an intelligible message all the same.)
Now suppose that a living cell needs a particular protein to form part of its outer membrane. In its central nucleus, the cell needs to store the instructions for making this protein. But how does it do this? Since each protein is made up of a string of smaller molecules (amino acids), the cell uses a code to store the instructions that tell it in what order the amino acids must be pieced together. Then when it needs that protein, it calls up that message, reads it, and makes the needed protein.