Self-talk is the voice in your head that almost never shuts up if it has a chance to talk. It is usually quiet when you are listening or reading, but even then it may interrupt. Unwelcome interruptions are called attention deficit. By itself, attention deficit is not a mental health problem. It happens to everyone.
The voice, of course, is just your thinking. You may believe that the thinking is you; that the thinking expresses your personality. But, not everything you are thinking is you. It is a combination of your personality, what you hear and read, what you think about what you see, what you are influenced to think about by other people, and your knowledge and memories.