From Sandboxie website: "Sandboxie runs your programs in an isolated space which prevents them from making permanent changes to other programs and data in your computer".
Basically what that means is that when you run programs or files under Sandboxies supervision, all changes while doing so remain isolated from the rest of the system. For example, if you are browsing and you go to a site that starts downloading malware, the infection will stay within the sandbox and once you delete the sandbox, the infection is gone. Since you were using SBIE, the system remains intact.
Sandboxie is a lot more than a browser in a sandbox. You can sandbox any file or program and run it sandboxed but most people when they start using SBIE, they use it to sandbox the browser. Thats how most people use Sandboxie at the beginning. I think that is fine but users that use SBIE only to sandbox the browser are not taking advantage of Sandboxies full capability.
Myself, I use it to sandbox just about any program or file that I run. I dont care what it is, if its gonna run, its run sandboxed.
I always say that I feel lucky to have discovered Sandboxie, at the same time I feel that the wisest decision that I ever made with respect computers is deciding to use it.
You don't need to learn everything in one day, you can install it now and run your browser sandboxed. After a while, if you like the program you might start running your PDF reader or your video player and before you know it, you might become a full time Sandboxie user.