A second mortgage is another home loan taken out against an already-mortgaged property. They are usually smaller than a first mortgage.
The two most common types of second mortgages are home equity loans and home equity lines of credit (HELOC).
Like a first mortgage, your home is used as collateral for a second mortgage. Should a foreclosure happen, the first mortgage lender is first in line to get repaid. The second mortgage lender is repaid next.