It's a platform that you install on your own server, at home or on a VPS (Digital Ocean, etc).
The installation is a few simple CLI steps. From there everything is in the browser, including the admin. Each user, once logged in, can install whatever apps they want from the app store. Or they can upload their own if they want. This is safe because everything is containerized. Apps have to be specially designed (or retrofitted) for Sandstorm, though.
Is this a remote server where you can install and use web apps from third parties?
Or can you actually build a web app there so that users can employ it? Something similar to Digital Ocean.