It is the general idea of the paragraph, which is the main idea. Write this paragraph in a general and concise sentence, mainly to explain something.

The key word is to let people who read your paper know what you wrote from one word.

For example, you say website construction:

To build a website, we must first establish a home page, which contains the general framework of the whole website. It is the soul of the whole website and the window of the website. Therefore, it is very important to build a good homepage.

Then you can expand around this idea and write in more detail.

Keywords: home page/frame