It is the behavior used to communicate. Speech is one communication form. Sign language is another. Crying, using objects, using pictures, even falling asleep - all of these are behaviors we do, forms we use, to communicate. All of these are reasons why we communicate.
Content is the part of communication that deals with meaning.
It is noon. I am hungry. My voice rises at the end of the word, and I raise my eyebrows when I say it. I am using these forms to ask you if you want to have lunch with me. That is why I say that word to you in that way.