From Sterwiki

A secured loan (or mortgage) that is subordinate to another loan against the same property. More specificaly, the second loan in sequence.

In real estate, a property can have multiple loans against it. The loan which is registered with county or city registry first is called the first mortgage. The loan registered second is called the second mortgage. A property can have a third or even fourth mortgage, but those are rarer.

Second mortgages are called subordinate because, if the loan goes into default, the first mortgage gets paid off first before the second mortgage gets any money. Thus, second mortgages are riskier and generally have a higher interest rate.