‘Carry trade’ is where you borrow and pay interest in order to buy something else that pays higher interest, the difference (when it is working as planned) is called ‘positive carry’. Usually this is done in bonds or currency, for instance, if you were to borrow money on short term rates to finance longer term bonds. The interest being paid on the long term bonds minus the interest on the short term borrowing would be the ‘carry return’. In currency the Yen was a very popular carry trade currency as their interest rate was 0%. So you could borrow in Yen, buy something else (unfortunately this money often ended up in CDO’s) such as US Tnotes and keep the difference, the main risk being that one that the Yen would strengthen significantly meaning you couldn’t pay back the original loan.
How does this affect mortgages though?
NOTE: THIS IS NOT A SUGGESTION THAT YOU DO WHAT IS DESCRIBED HERE! THIS IS MERELY MAKING A POINT!