Please make at least a token effort to post your questions in the correct forum. This has nothing to do with SEO, advertising or promotion; it's a technical question relating to computer security.
A "sandbox" is, metaphorically speaking, a safe place to play. In computing terms, a sandbox is a run-time area that has limited, brokered access to outside resources. Theoretically, code running in a sandbox environment should not be able to affect code or data elsewhere on the machine. A web browser (or tab, in the case of some browsers) for instance, is supposed to be a sandbox; things that happen on a web page should not affect anything other than what's happening in the browser. However, you may have noticed that things don't always work out that way. Flaws in browser code and in plugins like Flash, ActiveX and Java often allow malicious code to run in the browser and infect the rest of the machine by exploiting those flaws.