You can deduct certain expenses and other adjustments from your total income to get your taxable income. These deductions reduce the amount of income you pay tax on, so they reduce your overall income tax. The more deductions that apply to you, the less your taxable income becomes. Deductions from taxable income are listed on lines 244 to 256 of the T1 form below.

Link to 2017 T1:

https://www.canada.ca/content/dam/cra-arc/formspubs/pbg/5006-r/5006-r-fill-17e.pdf