What does it mean for a negative to be decreased by a negative?

It becomes less negative

To explain. Lets take a negative, eg -5.
If we decrease it by 1, it becomes -6 (where 1 is positive)
Thus if we decrease it by -1, it becomes -4 (as the -1 is negative)

Note that the final number could be positive,

For example, if you decrease -5 by -6, you get +1