Put simply, the greater the frequency of your mortgage repayments, the more you will save in interest over the life of your mortgage.
If you make your mortgage repayments weekly or fortnightly, you are paying down the principal amount faster than if you were to make monthly mortgage repayments. This is because the interest calculated has a lesser amount of time to accumulate. Interest is calculated on the principal balance, so with less principal owing, there's less interest payable.