A piggyback loan is simply a second mortgage that is created at the same time as the first mortgage. Borrowers often get two loans at the same time to avoid paying mortgage insurance. Thus, the nickname came about "piggyback", one loan on top of the other.
A piggyback loan is also called a second mortgage or a HELOC (Home Equity Line of Credit). Though often used to get rid of Mortgage Insurance, they can also be used for a myriad of other reasons that are advantageous to the borrower.