What is Tax-deferred?
Tax-deferred means tax is postponed rather than cancelled. Money usually goes in before tax, or earns a deduction, and grows without yearly tax, but withdrawals are taxed as income when they come out. RRSPs, traditional 401(k) and IRA contributions, and UK pensions work broadly this way.
Why Tax-deferred matters
Whether deferring tax helps depends heavily on your tax rate when the money goes in compared with your tax rate when it comes out.