ANSWER: In mortgage slang it refers to a financing option you could choose to avoid Private Mortgage Insurance (PMI). The 80 refers to an 80 percent first-lien mortgage. The first 10 refers to a 10 percent second-lien mortgage and the second 10 refers to the required down payment of ten percent.
The advantage to this type of financing has been somewhat lost as of 2008 when congress made PMI tax deductable. It is important that your particular situation is examined and that your mortgage is structured in order to meet your goals.