Fresh Start was introduced witha the original Creators Update last spring, and can be found within Windows Defender. So, what exactly does Fresh Start do? Microsoft provides the following overview:
Fresh start will remove all apps that don't come standard with Windows, including other Microsoft apps such as Office, third-party antivirus software, and desktop apps that came pre-installed on your device. It will also remove all other Windows Desktop apps installed by your device’s manufacturer, such as your manufacturer's apps, support apps, and drivers. It will keep only Microsoft Store apps installed by your manufacturer.
Microsoft also cautions that your digital licenses for apps (i.e. Microsoft Office) will be removed, so you'll want to make sure that you have all of your license keys backed up and handy when it comes time to reinstall all of your important apps. You will also need access to your original installation media (USB thumb drives, CDs/DVDs, etc.), but that's a given.
If these caveats are manageable, you'll want to start the process by going to Settings --> Update and Security --> Recovery. From there, you'll need to go to “More Recovery Options”. If you've made it this far, you should see a link that says "Learn how to start afresh with a clean installation of Windows". Clicking on the link will then take you to Windows Defender, where you'll be able to start the process.
You'll next want to click on the “Get Started” button, after which you will be able to initiate the process. After displaying a list of all the installed apps that will be removed, the Fresh Start will then initiate, and your copy of Windows 10 will automatically be upgraded to the most recent release -- in this case, the Windows 10 April 2018 Update.
Once the entire process has been completed, you will have a newly refreshed and up-to-date version of Windows 10, but you will then have to complete the process of installing all of your device drivers (which can easily be accomplished via Windows Update) and reinstalling all of your apps.
This Fresh Start tool should come in handy given that some people have encountered software compatibility issues with the latest versions of Windows 10, like the high-profile issue with Avast that we've reported on in recent days. Clearing out all of your cruft beforehand -- while keeping your critical data intact -- may be one of the smartest ways to avoid these speed bumps while receiving a squeaky-clean Windows 10 install in the process.