The IRD is a compensation charge that may apply if you pay off your mortgage prior to the maturity date, or pay the mortgage principal down beyond the amount of your prepayment privileges.
The IRD is based on:
The amount you are pre-paying; and,
An interest rate that equals the difference between your original mortgage interest rate and the interest rate that the lender can charge today when re-lending the funds for the remaining term of the mortgage.
Most closed fixed-rate mortgages have a prepayment penalty that is the higher of 3-months interest or the IRD. Most variable-rate mortgages do not have IRD penalties.
Here is a calculator that let’s you estimate your mortgage penalty. Edit the figures in yellow and press Tab after each edit.