Sandbox is the program area and set of rules that programmers need to use when creating Java code (called an applet) that is sent as part of a page. Since a Java applet is sent automatically as part of the page and can be executed as soon as it arrives, the applet can easily do harm, either accidentally or as the result of malicious intent, if it is allowed unlimited access to memory and operating system services. The sandbox restrictions provide strict limitations on what system resources the applet can request or access. Essentially, the programmer must write code that "plays" only within the sandbox, much as children are allowed to make anything they want to within the confined limits of a real sandbox. The sandbox can be conceived as a small area within your computer where an applet's code can play freely - but it's not allowed to play anywhere else.
Sandbox programs can sometimes be integrated into some antivirus software.