That means that if your phone is running iOS 17, you can update it to iOS 18. You might not get all the features, though.
Apple is bringing major improvements to its smartphone operating system with iOS 18 – it opens the next chapter in AI, enables RCS, brings other communications features (text effects, emoji reactions, etc.), a password manager and improved customization features. And there is some good news: all iPhones that are currently running iOS 17 can be updated to iOS 18.