What does Sandbox mean?
A sandbox is a style of game in which minimal character limitations are placed on the gamer, allowing the gamer to roam and change a virtual world at will. In contrast to a progression-style game, a sandbox game emphasizes roaming and allows a gamer to select tasks. Instead of featuring segmented areas or numbered levels, a sandbox game usually occurs in a “world” to which the gamer has full access from start to finish.
A sandbox game is also known as an open-world or free-roaming game.
Techopedia explains Sandbox
Sandbox games can include structured elements – such as mini-games, tasks, submissions and storylines – that may be ignored by gamers. In fact, the sandbox game's nonlinear nature creates storyline challenges for game designers. For this reason, tasks and side missions usually follow a progression, where tasks are unlocked upon successful task completion.
Sandbox game types vary. Massive multiplayer online role-playing games (MMORPG) generally include a mixture of sandbox and progression gaming and heavily depend on emergent interactive user gameplay for retaining non-progression-focused gamers. Modern "beat 'em ups" and first-person shooters have delved more deeply into the sandbox realm with titles like the "Grand Theft Auto" series, "Red Dead Redemption," "Assassin’s Creed" and others, allowing gamers to run and gun wherever the mood takes them.
In spite of their name, various sandbox games continue to impose restrictions at some stages of the game environment. This can be due the game's design limitations, or can be short-run, in-game limitations, such as some locked areas in games that are unlocked once certain milestones are achieved.