what does 3d mean in javascript

1/* JavaScript shorthand -=
2-= is shorthand to subtract something from a
3variable and store the result as that same variable.
4*/
5
6// The standard syntax:
7var myVar = 5; 
8console.log(myVar) // 5
9var myVar = myVar - 3;
10console.log(myVar) // 2
11
12// The shorthand:
13var myVar = 5;
14console.log(myVar) // 5
15var myVar -= 3;
16console.log(myVar) // 2
17