It is the letter you will receive from IRCC if you have been approved for a: study permit, work permit, or Super Visa (only if you are from a country that does not require a visitor visa).
It is sometimes called the Correspondence Letter, Introduction or Introductory Letter. Officially, it is called the Port of Entry (POE) Letter of Introduction. The Letter of Introduction is not a study or work permit.
You must show this letter to officials at the port of entry when you arrive in Canada to get your study or work permit.