It's a "magical" disc with space assigned to your account with Google, or with Apple, for instance. You can save important info, pictures, etc. on it. It is supposed to be secure, and probably is as secure from hacking as any device is these days. You can put things in the 'cloud' and take them out, share them with friends or not, and so on.
So from our user perspective, it is like a USB drive or CD, but it's more accessible to you from any device you have, and you don't have to worry about it being lost, broken or stolen by someone who is not you.
I just got an Apple phone, so am learning some of this stuff for the first time. Google is similar. It appears that with both Google and Apple clouds, the stuff you put in the clouds is available to you on other google devices if you are using Google cloud, or, if you have Apple, other Apple devices you use. I haven't seen how to get something from the Google cloud to the Apple Cloud, but there probably is an app for that...
That exhausts my knowledge as of today!