It's kind of a website, though the main part of the website depended largely on Adobe flash, which is no longer supported by internet browsers, so now there's an app you have to download which is basically a private browser.
(Yes, it's confusing.)
But that aside, it's basically a thing that lets you make and share video games, with no programming required. You normally have to pay to get the full thing, but right now (due to all the stuff mentioned above, and maybe because they're going to rework the site) I believe you can get all the content free.
