In computing, a sandbox is a free app that lets you play any video game in your browser without downloading it

Explanation

A sandbox is an isolated environment where you can run untrusted code without it touching the real system. It blocks access to the file system, network, and hardware so that malware or buggy code can't spread. Examples include sandboxed browser tabs, virtual machines, and Docker containers. The name comes from a child's sandbox: a contained space where you can play without making a mess of the house.