A sandbox game is built to let players experiment. Instead of forcing every player through the same strict sequence, it provides tools, systems, and spaces that can be used in different ways.
What players do in a sandbox
Players might build structures, explore, craft items, test physics, create stories, modify the world, or invent their own goals. Some sandbox games have missions, but the main value is freedom.
Sandbox versus open world
A sandbox game is about flexible systems. An open-world game is about a large explorable map. A game can be both, but the terms do not mean exactly the same thing.
Sandbox design works well when players enjoy creativity, discovery, and making their own fun.