A sandbox is an isolated environment that behaves like the real system but is kept separate from it. Nothing done inside it affects real customers, real stock or real invoices. It is the software equivalent of a practice kitchen: staff can make mistakes, learn and experiment freely, and nobody sitting in the restaurant ever notices.
It lets you try a new automation, rehearse a data migration or train staff without worrying about breaking something. A change that works in the sandbox is not guaranteed to work live, since volumes and real-world data differ, but it catches most obvious problems early, when they are cheap to fix.