"The Sandbox" of Bungie
In the particular case of Bungie, "The Sandbox" refers to all the things with what a player can directly interact, for instance in Destiny. That is also what Jon Weisnewski, working at Bungie, is saying in his resume :
The Sandbox team is responsible for controller input, the "feel" of moving through the world and performing actions, player abilities, grenades, melee, vehicles, weapons, armor, and custom sandbox content to support activities like Raid Relics, Scorch Cannon, etc.
The Sandbox term in gaming
The Sandbox term usually corresponds in gaming to an area in which the user is free to do what he wants.
If refers to the notion of sandbox game :
A game wherein the player has been freed from the traditional structure and direction typically found in video games, and is instead given the ability to choose what, when, and how they want to approach the available choices in content. The term is in reference to a child's sandbox in which no rules are present and play is derived from open-ended choice. While some sandbox games may have building and creation aspects to their gameplay, those activities are not required. Sandbox games usually take place in an open-world setting as to facilitate the freedom of choice a player is given.
It is called sandbox because you are able to do whatever you want there, like in a real sandbox: you are the one building your adventure. You are in a world where you can walk around and go for your own objectives. There is a lot of existing exemples of sandbox games, such as Minecraft, The Elder Scrolls games, Kerbal Space Program, Garry's Mod, Don't Starve, or EVE Online.