If something is self-explanatory, you can understand it without extra help. The thing itself is clear enough. Signs, diagrams, and simple instructions are the usual examples: a picture of a man and a woman on toilet doors is self-explanatory. People use this word at school and at work, a little more in writing than in street talk. Though actually, calling something self-explanatory is often one person's judgment. What is obvious to the writer may not be obvious to you — and when a teacher or a boss says "it's self-explanatory," it can land as "you should not have asked." The word is almost always about things, not people. We do not usually call a person self-explanatory.