It's generally a C (not C++) syntax of "operate and assign back" (as I call it) operator.
1 2 3 4
a += 2;
b -= 2;
c *= 2;
d /= 2;
are equivalently the same as
1 2 3 4
a = a + 2;
b = b - 2;
c = c * 2;
d = d / 2;
But you need to note that operators in C++ can be overloaded. The equivalences I mentioned above is applicable for primitive data types. If the variables are objects it may or may not mean the same thing.