This is also called a “demand to file” notice. What it is, is a request from the CRA to file late taxes. Once a business or person receives such a letter they are obligated to comply by quickly paying the back taxes or contacting them to work out a payment plan. This generally occurs when a person or a business fails to file for a number of years, for whatever reason, and the CRA finds this in their records.
The notice will specify which tax years are delinquent in addition to the amount of interest and fines attributed to the late payments. In this scenario, having a professional accountant work on this is the best way to handle such a situation. The reason for this is that the accountant will have the knowledge and experience specific to handling back taxes. This also translates to getting things correct thus avoiding any potential repeat requests for the given tax periods in question.