An RRSP contribution is deducted from your taxable income before federal and Ontario tax get calculated, dollar-for-dollar, the same mechanic as any other above-the-line deduction. That means the refund you generate depends on your marginal rate, the rate on the last dollars you earned, not some average rate across your whole salary. A dollar contributed while you're near the top of the Ontario Surtax range saves meaningfully more tax than the same dollar contributed by someone earning half as much.
Your contribution room isn't reset each year at a flat number, it accrues at 18% of your prior year's earned income, capped at a CRA dollar maximum that only binds once you're earning well into six figures. Whatever you don't use carries forward indefinitely, so if you've never maxed out before, your actual available room is very likely higher than 18% of last year's salary alone, worth checking on your CRA My Account before you assume a number.
The tax saving doesn't always show up immediately. Unless your employer runs a payroll RRSP deduction program that adjusts withholding right away, the saving shows up as a bigger refund, or a smaller balance owing, when you file your T1 return the following spring. Either path gets you the same total saving over the year, it's purely a question of when you see the cash.