The tax rate applied to the last (highest) dollar of income earned, within a progressive tax system where different portions of income are taxed at different rates.
Canada's tax system is a series of brackets, not one flat rate on everything you earn. Your first chunk of income is taxed at the lowest bracket rate, the next chunk at a higher rate, and so on. Your marginal rate is just the rate on that top chunk — it does not mean your entire income is taxed at that rate, which is one of the most common misunderstandings in personal finance.
Someone might be in a 30% marginal bracket, but their average (effective) tax rate across their whole income is often noticeably lower — maybe 20% — because the earlier, lower brackets pulled that average down. Your marginal rate is what matters most for decisions like an extra RRSP contribution, since that's the rate being saved on the next dollar contributed.