[afortiori]

It's a financial contract that pays off if there is a default. Although mortgage holders might use it to hedge their position in case of default, some may buy them simply speculate. From Wikipedia:

A credit default swap resembles an insurance policy, as it can be used by a debt holder to hedge, or insure against a default under the debt instrument. However, because there is no requirement to actually hold any asset or suffer a loss, a credit default swap can also be used for speculative purposes and is not generally considered insurance for regulatory purposes.

[Perdogg]

I understand now.