It's difficult to tell from this forum and not being able to see your individual tax situation, but the only thing that should be showing up on Line 20800 is a Registered Retirement Savings Plan (RRSP) contribution. This could be a slip that has been entered manually, or (most likely) if you used the CRA AutoFill my Return (AFR) service you may have had an Unused RRSP Contribution carried forward and automatically entered by the import from CRA.