One of the original meanings of "coffer" is a strong box or chest. Probably people used it to store precious things. We now often use "coffers" to refer to a nation's or individual's financial reserves (in other words, the money they have available to them).
Other examples of how the word might be used (with the same meaning as the example you gave):
Christmas put a big dent in the family coffers.
With the current global financial difficulties, many governments will have to dig into their coffers to help their people.
I hope this helps.