After prop 13, the state had to have a vote in order to issue "General Obligation" bonds. This was the method used to finance things like streets, curbs and gutters, schools and parks in new developments. In order to promote new developments they passed the Mello Roos bill to provide financing for these items. They are a general obligation of the new homes that were built.
That is what they are how the affect you.
For some reason the typically tend to double the tax rate to about 2.2%.
So if you are buying an old property, there most likely is no Mello Roos taxes. If you are buying a newer property you better ask or assume you have it.