Junior Mortgage
  
You've got junior prom and senior prom. You've got junior senators and senior senators. You've got Robert Downey Jr. and Robert Downey Sr.
And you've got junior and senior mortgages. Junior mortgages are considered subordinate debt, as compared to the senior variety. In the case of default, the junior mortgage will only get paid back after the senior mortgage gets repaid first.
The distinctions basically designate where in line a creditor stands when a bankruptcy happens. Senior mortgage...front of the line. Junior mortgage...somewhere behind senior mortgage.
You buy a house with a mortgage. After a while, you take on a second mortgage to pay for the ashram you want to build in the backyard. That second mortgage is the junior mortgage.