Writing Tools is one of the main Apple Intelligence features that's available in the iOS 18.1 update released on October 28. As the name suggests, this toolset is meant to help you write, but it can also rewrite text that you think could be cleaned up or made better by adopting a different tone.
The Writing Tools Rewrite feature can refine what you've written, but it can also be used to change the tone of your writing. Simply select some text and choose the base Rewrite option, and Writing Tools will generate a version of your text that's more refined and elaborated, but there are also Friendly, Professional, and Concise styles that you can choose from. The standard Rewrite option is a sort of mix of all three styles.
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 has yet to mature, and rewrites might not always be up to the standard you were hoping for. Apple has work to do to refine the experience, but it's only going to get better with time.