RRSP is a second largest tax shelter in Canada. what it does is to REDUCE your "taxable income" in the present. and DELAY your tax payable in the future.
how it works? let say at ur working age you have a 30% average tax rate based on your income of 50k, by contributing part of your income to RRSP, it effectivly reduces your "taxable income" to 40k, therefore, your average tax rate decreases to 20%. you pay less tax at your working age now.
as time goes by, at age 65, you are retired. your income drops significantly. the only income comes from withdraws of your RRSP. at that time your average tax rate is lowered becasue you have much less income than at your working age.
in real practice, there's more than one aspect you need to consider in terms of RRSP. i.e. investment income related to RRSP, Home Buyer's Plan, etc.