Ethical theories attempt to give general, abstract accounts of ethics. Ethics, in turn, concerns the consideration we give to other people in both thought and action. So we are looking at general, abstract accounts of why some actions are right and others are wrong or why some people are good and others are bad.
Not every way of being right or wrong, or good or bad, counts. Some people are better than others by virtue of being stronger or better looking, for instance. What an ethical theory seeks to describe, by contrast, is the consideration we have to give to others in order to be a good person.
That said, some ethical thoughts are about ourselves and not others. “I couldn’t do that” can be a mark of an ethical person even if “that” doesn’t have any significant effect on others; think about breaking a promise made to someone who has died. In any event, the central cases involve the effects of our behavior on others.
What are philosophical theories of ethics, you might ask? They involve arguments: a structured series of assumptions (we call them “premises”) that lead to (we often say “entail”) a conclusion. There is a lot to be said for this way of approaching the subject. Among other things, it can make it easier to discuss topics on which people disagree. When you put your argument down on a piece of paper or a blackboard everyone can focus on exactly where the disagreement lies. It’s usually, though not always, a productive alternative to getting upset with one another. And ethical disagreements are one thing that people make people very upset.
That said, this cannot be the only way to learn about ethics. Arguments are about the relationship between premises and conclusions. You can be very good at analyzing this sort of thing while having no idea what you’re talking about. In order to genuinely have ethical knowledge, you need to be able to evaluate the assumptions. Philosophical reflection gets you part of the way there. In particular, it encourages a degree of consistency that most of us do not naturally have. But that will only get you so far if you don’t understand other people very well.