SideQuest is an application you install on your computer that makes sideloading content onto your Quest headset fairly easy (there are other ways, but unless you are pretty tech savvy, this is the easiest way). It does take a bit of work to set up in the first place, and you’ll have to register as a developer with Oculus, but once you’d done that you can download and install items from their store. Many are free, while others have a cost. Check out the huge number of available apps at https://sidequestvr.com/
UploadVR has an easy-to-follow guide on how to get set up and using SideQuest. Try not to be intimidated by the number of steps – it really is straightforward if you just walk through them one-by-one.
Once you have sideloaded apps onto your headset you won’t find them in your main library. Instead use the dropdown box in the upper right (which will say “All” as the default) to select “Unknown Sources” at the bottom of the list, and your sideloaded apps will then appear.
SideQuest is also serving as the directory for apps for the newly-launched App Lab, which enables you to download apps directly into your headset without going through all the steps of sideloading. App Lab apps will show up in your main app library. Click here for more information about App Lab.