A PR card is a Permanent Residency card given to Canadian permanent residents. When travelling it is important that you hold a valid PR card as you will need it to prove your permanent resident status when you enter Canada again.
Most PR cards are valid for 5 years, the validity of your card will be printed on the front. When your PR card expires you must renew it before you travel out of Canada. If you are currently outside of Canada and your card is not valid then you will need to apply for a Permanent Resident Travel Document (PRTD) to enter Canada.