To answer that question we need to first ask what subprime lending in general is.

A subprime loan is a loan made to a high risk individual. Because the individual in question is high risk, the lending institution must charge a higher rate of interest in order to justify making the loan.

Now you might be wondering what makes an individual a "high risk?" The answer may relate to a number of things, but it often involves defaulting on previous loans, a bad debt/asset ratio (too much debt already), little experience paying off loans, or a number of other factors. Basically anything that might lead a bank to think someone would not pay back their loan could lead the bank to demand a higher rate when lending to that person.

So, a mortgage is just a special case of a subprime loan. It is a typical subprime loan that is used to finance the purchase of a home.