When speakers/communicators are being considered there are at least two skills that are of concern: writing and speaking. Although those aren't the only methods to communicate they are two of the most common. Those have to do with our abilities to encode messages into some form for transmission. If we are thinking of audiences in traditional terms then there are two additional skills that involve decoding. We decode by reading and listening. These ideas are clearly discussed by David Berlo in his book, The Process of Communication.
Before successful communication takes place we experience thought or reasoning. This step is necessary for both the writer/speaker and the reader/listener. These elements deserve our thought and attention if we hope to be able to communicate what we intend to and be understood by any audience.
It makes sense then, that we constantly improve our writing and speaking abilities and that we spend considerable time and focus on the reasoning we use and expect the listener to be able to follow. If this were an easy process, then nearly everyone would be successful communicators.