What is the "sandbox"?

In the article Bungie 2.2.0, Bungie released information on their upcoming update. The article mentions there will be updates to the Sandbox:

Destiny Update 2.2.0 will deploy on April 12th, 2016.

From here onward, we’ll refer to it as the “April Update.” As promised, it will have something for every type of Guardian. Here are some of the highlights to look forward to:

New PVE Challenges
New and Updated Gear
New Quest featuring the Blighted Chalice Strike
Increased Max Light and Rewards
Sandbox and Crucible Updates
And more…

I have seen this term multiple times in regards to update releases.

What does the term Sandbox mean?

terminology destiny

Best Answer

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.

Ref

Basically in a sandbox game you don't have a linear mission path you need to follow (or if you do, you also have the sandbox option to explore), but instead you can do whatever you want, and whatever the game limits maybe. Prime examples are GTA and Minecraft.