It's a privilege that allows you to pay up a portion of mortgage principal monthly or annually without receive any penalty.
You can use this mortgage calculator to investigate how much can you save with a pre-payment plan.
I also have an excel file for calculating Variable Mortgage, which is an excellent tool to understand how much better off / worse off to get a fixed/variable rate at the current time. http://www.fcac-acfc.gc.ca/iTools-iOutils/MortgageCalculator-eng.aspx
Broker A:
4 year 3.5% rate for quick closing
Broker B:
5 year 3.79% rate with 15% pre-payment privileges
5 year 3.69% rate with 5%/5% pre-payment privileges (no frills)