Definition
Android, iOS, and the browser each run an app in its own sandbox. The app gets its own files. It cannot read another app's data or change system processes unless the platform grants that right. A browser engine such as Chrome or WebKit adds a second sandbox around pages. Getting code to run inside the box is one step. Leaving the box takes a separate flaw.
Relevance
A Unity player on Android, iOS, or WebGL runs inside the platform sandbox. Code injected into the game starts with the game's files, the game's memory, and the permissions the player granted. Reaching another app or a system process means escaping that sandbox.
Example
A library loaded into your Android game can read the game's memory and its files. Another app on the same phone keeps its files in a different sandbox, so the library cannot open them. Reaching those files takes a second flaw that escapes the game's box.
Connected
Synonyms
App sandbox