Why does Next.js prefix the app.js with an underscore?

Question

When I create a Next.js project with the npx create-next-app command than there is an _app.js file under the pages folder. In which case do I need to prefix a file with an underscore in a Next.js project?

question from:https://stackoverflow.com/questions/65857281/why-does-next-js-prefix-the-app-js-with-an-underscore