Answer:
The main idea of any type of text (spoken or verbal), including a lecture, is the main, most important point the writer/speaker is making. In one text, there can be multiple main ideas. You can easily recognize the main ideas of a lecture by looking at its title and subtitles. For example, if you are listening to a lecture about the central nervous system, the main points could be the central nervous system itself and its most important parts, which will be represented by titles and subtitles.
When you're taking notes, you should write down the most important things the lecturer is saying. This includes the things that are closely related to the main ideas, such as definitions and explanations. If you are unable to write everything down (which is usually the case), you should focus on these main points and ignore the details, because it's more likely that you won't need them.
