Organizing routes into folders that don't affect the URL at all.
A folder name wrapped in parentheses — (marketing), (auth) — creates a Route Group: a way to organize related routes together in the file system without that folder name ever appearing in the actual URL. It's purely an organizational tool, invisible to anyone visiting the site.
The main practical use is applying different root layouts to different sections of the same application — a marketing site layout and a dashboard layout can both live under the same app directory, each inside its own Route Group with its own layout.tsx, without needing one shared top-level layout that awkwardly tries to serve both. Two Route Groups resolving to the exact same URL path is a genuine conflict Next.js will flag, since the grouping doesn't change what URL a page actually lives at.
What you'll walk away knowing