Every article or piece of writing (or every communication) has a main idea. This is usually expressed in one or two sentences. The main idea is what your paper is about -- in a nutshell. In the descriptive paper, however, the whole paper can be the main idea -- dominate impression. Here are some examples of main ideas:
Self-driving cars are safer than standard models because they operate more effectively and eliminate accidents caused by human error.
In "A Hanging," George Orwell shows that capital punishment is not only brutal but also immoral.
Once I saw someone cheating and did not speak out.
The main idea has only one focus: it always points to one concept. As a writer, you want -- need -- to provide a clear understanding of your topic.