Predictive Text is an input technology that facilitates typing by suggesting and inserting the words as you type. When enabled, it reduces the number of keys that the user has to type while writing. This feature you can see enabled on mobile, and on some sites like Gmail, Outlook.com.
If you are wondering, Windows 10 also comes with the Predictive Text feature, but that is enabled by default for tablets. If you are using the Windows 10 desktop mode, you can enable it by following the steps below.