The tuition amount is first used to reduce the amount of taxes you owe, meaning you can not only reduce the amounts payable at the time of filing but the amounts you paid at source, withholding tax, through your employer.
please see: https://turbotax.community.intuit.ca/questions/1190116-frequently-asked-questions-about-transfering-...
or the CRA site: http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/323/crry-frwrd-eng.htm...
