Writing Tools is one of the main Apple Intelligence features that's available in the iOS 18.1 update that Apple released on October 28. As the name suggests, this toolset is meant to help you write, but it also has the ability to summarize existing text in various apps.
You can select text and use the Writing Tools summary options to automatically distill a longer piece of text into something more easily digestible. It's particularly useful for long web articles, PDFs, or even your own notes if they're on the lengthy side.
In an app like Notes or Pages, you can simply select text and then tap the Writing Tools option. Apps like Notes also have a Writing Tools interface built-in, so you can also get to Writing Tools from the in-app toolbar. The options for summarizing text include Summary, Key Points, List, and Table.
You can use Writing Tools anywhere there's text on your iPhone, as it's a system-wide feature. You can use it in third-party apps, too. Just select the text and then choose the same Writing Tools option in the contextual menu.
It's worth mentioning that Apple Intelligence remains in beta, and summaries can sometimes be overly brief, or fail to include key details. Clearly Apple has work to do to refine the experience, but it's only going to get better with time.