If you have a mortgage on your house, a car loan or any other form of debt, you can account for it by adding a Loan - so that your overall wealth calculation does not miss the money you owe.
When you add a real estate Investment, you have the option to record a mortgage. A property mortgage is a special kind of Loan as it is linked to a specific property, which represents its collateral. If you add one, it will be automatically recorded among your liabilities.