A sandbox is an isolated place where a program can run with limits on what it can access. It helps keep the program's activity separate from other parts of the computer.
Imagine letting someone try a toy in a fenced play area rather than giving them the keys to the whole house. A coding tool might use a sandbox to limit which files or services it can reach.
A sandbox reduces some risks, but it is not a guarantee that nothing can go wrong. The limits depend on how the environment is set up.