Your credit score is a measure of your financial health. According to the Government of Canada, your rating “indicates the risk you represent for lenders, compared with other consumers…The credit-reporting agencies Equifax and TransUnion use a scale from 300 to 900. High scores on this scale are good. The higher your score, the lower the risk for the lender.” Thus, a higher rating will typically secure a better mortgage rate, since you’re considered to be more likely to make your scheduled payments.
If you have a poor rating, do some damage control before you apply for your mortgage. For more information, read this.