When we first begin studying a new subject, we see a vast sea of facts, and we may begin to sink. The key to staying afloat is to look for the main ideas and to let the facts fall into place. That may be obvious to you, but many people do not know how to do it. Those are main ideas. In a science course, the main ideas are usually theories, methods, or processes.
Theories are specific, testable statements about how things happen.
We might talk about the theory for the origin of comets, for instance. If you look for theories, methods, and processes and are able to discuss them, you have conquered the heart of the subject. The main ideas must be supported by facts, and many students have trouble with all the facts in science courses. They even try to memorize the facts as if they were memorizing the kings of England. The key is to link the facts to the main ideas.