What is a great communicator?

In my opinion, no matter how good at what you do, if you can't communicate with the fellow team you aren't going anywhere.

As others commented, learn to understand the listener, their background, their interest in asking the particular question and communicate clearly what's required. Example scenario would be answering the same question asked by the CEO vs a colleague/intern in the team.

It does take a lot of effort to do this, and the statement is absolutely true. There's a work to be done to be a great communicator. (I'm in the progress of doing it).

I'm not sure if that's the kind of answer you are looking for but it makes me think of people who know things deeply and are able to adapt their speech to their audience.

If we take an example of a software developer, if your colleague or non-technical family member asks you a question, you won't answer the same way. But because this "successful professional" knows the ins and outs of his profession, he is able to communicate clearly the same thing to different people.

At least, this is what this quote made me think of.

Fidel Castro?