An SSL Warranty is issued for the very unlikely event that an SSL Certificate is issued to a site that later commits fraudulent transactions. This almost never happens, but to assure any site visitors that we take full responsibility for issuing our certificates, there is insurance for any transactions made on a site bearing an SSL Certificate. The amount that these warranties cover differs based on the SSL Certificate in question.
If you make a transaction on an SSL-protected site that turns out to be fraudulent, then you can have the transaction refunded using the SSL Warranty. This is a way that we protect you at every stage of your online interactions - making sure that nobody steals your information, and even if something goes wrong, we'll still look out for you. That's what our insurance is for.
It's just one of the ways we try to make the internet a little bit safer. If someone suffers an undeserved loss on a site that we secured with an SSL Certificate, we want to do the right thing.