"Main idea" is the most important concept or point that someone is trying to convey in a piece of writing or a speech. It is the central theme or argument that the writer or speaker is trying to get across to their audience.

Here are a few examples of how you can use the phrase "main idea" in Engpsh, along with their translations in Chinese:

"What is the main idea of this paragraph?" (这一段的主要意思是什么？)
"Can you summarize the main idea of the article in one sentence?" (你能用一句话概括文章的主要意思吗？)
"The main idea of the presentation was to introduce the new product." (演讲的主要意思是介绍新产品。)

The phrase "main idea" is a common term used in Engpsh language education, as it is an important concept for students to understand when analyzing and summarizing texts. It is also commonly used in academic and professional contexts to refer to the central point or argument being made in a written work or presentation.