When you incur a "Capital Loss" in the tax year, you can choose to apply that loss to any Capital Gains you incurred in the previous three years.
Or you can Carry it Forward, and apply it to any Capital Gains you may incur in the future. If you choose to "carry it forward", when you receive your Notice of Assessment (NOA) it will state on there you have XXX amount of Capital Losses to apply in the future.
The following TurboTax Tip explains this further: https://bit.ly/2HV6q17
You first need to enter sales or dispositions of your stocks. Do so by going to Income > Investments > Capital Gains & Capital Gains Deduction Profile. Then check off the box "Sold stocks, bonds, real estate ,etc ", and continue to the Capital Gains page to make your entries.
When you do the Review using TurboTax Online, in the Tax Savings Opportunities section, click Explore next to "Any losses not applied to the three preceding years can be carried forward and applied against future years", and then "Applies to me". You can then continue to the page "Net Capital Loss Carrybacks". Here you can see how much of a Net Capital Loss you have incurred and make your choice to carry it back, or forward.