This is mainly a convenience thing, with a limited niche of Windows users that might be interested. The short version is that win10's browser, Edge, can use a protected mode with better isolation from the rest of Windows than you'll find using 3rd party browsers. For an individual user, these extensions simply make it quicker to open a protected mode window in Edge -- you can go right from Chrome or Firefox to that Edge window rather than starting that protected mode in Edge normally via the Taskbar.
Using that protected mode in Edge is a bit limited, e.g. the Favorites list doesn't cross over, and to make it more useful Microsoft recommends using it mainly in large companies that can maintain a listing of safe web sites, where Edge will automatically open a protected mode window when, according to that listing, you're visiting a site that isn't safe. It's still useful, working for the individual user, but you have to decide what's safe & when to use it, e.g. you might want to use that protected mode to visit a site you're afraid might not be 100% safe.
That protected mode in Edge goes beyond the isolation you'll find in other browsers, using part of Microsoft's Virtual Machine technologies, and that means that Defender Application Guard has to be installed via Control Panel -> Programs and Features -> Turn Windows features on or off. It also means that it requires win10 pro or Ent., and since it uses some tech from Microsoft's Hyper-V Virtual Machine stuff, it requires virtualization be turned on in the BIOS, and will [like win10's upcoming sandbox] break VirtualBox VM software. The 2 new browser extensions require a new win10 app that does the actual work of opening that protected window in Edge.
SO all told you need virtualization turned on in the bios, Windows Defender Application Guard installed, then optionally one or both extensions plus the new app. [And it all seemed to work in win10 1809 until I tried running a V/Box VM, found it wouldn't run, & promptly uninstalled everything.] Note that the link for the win10 app in the Neowin article does not work -- nor will searching the store. After installing the Chrome extension that web page had a good link for the app.
docs.microsoft[.]com/en-us/windows/security/threat-protection/windows-defender-application-guard/wd-app-guard-overview
neowin[.]net/news/microsoft-releases-windows-defender-application-guard-extensions-for-chrome-and-firefox
chrome.google[.]com/webstore/detail/application-guard-extensi/mfjnknhkkiafjajicegabkbimfhplplj
wdage.azurewebsites[.]net/
howtogeek[.]com/357937/how-to-enable-windows-defender-application-guard-for-microsoft-edge/
bleepingcomputer[.]com/tutorials/how-to-enable-cpu-virtualization-in-your-computer-bios/