To default on your home mortgage means that you stop paying it back and so the property goes to the bank that gave the buyer the mortgage.
Prior to the financial crisis, it was possible for people to buy a house without putting any money down. They borrowed the entire amount. So when the prices fell it no longer made sense for them to continue paying since they no longer had any value in the property. Sometimes they were not able to pay, and sometimes they were. When you default on a mortgage and you are able to pay, that is called a "strategic default".