The Plus Equals assignment operator will assign a
new value to the variable in front of it: whatever
the variable's current value is + the value after the
operator.

For example:

int NUM1 = 2;                /* NUM1 is 2  */

NUM1 += 5;                   /* NUM1 = NUM1 + 5;  So NUM1 is now 7. */

NUM1 += 2;                   /* NUM1 = NUM1 + 2;  So NUM1 is now 9. */