When Apple introduced iOS 10 for iPads and iPhones, they introduced a new feature that allows you to uninstall the standard (apple) apps that come pre-installed on the phone - eg. stocks, mail, newsstand etc.
The process to remove them is simple - just like removing any other app on your device, you press and hold on any of the icons on the home screen for a few seconds until they all start moving slightly, and have a little "x" in the top right corner above each icon. You then click the small "x" to remove the app
What isn't entirely clear or obvious at first is the process to get these apps back after you've removed them. Turns out the answer is very simple, you just need to download them again from the App Store!
For example, if you open the App Store and search for the "mail" app, the first result that appears is the default Apple Mail app. Click the download or "cloud" icon next to it to download it again, and away you go!