Your (PR Card Replacement) Permanent Residence Card is evidence that you have permanent residence status in Canada. You can also prove this with your record of landing in some cases (ex: only if you are driving through the border). If you lost your Permanent Resident Card, we can apply for a replacement.
If you are traveling outside Canada, you will need your Permanent Resident Card upon return to Canada to demonstrate your status. If you are from a visa-exempt country, it is still recommended that you travel with your Permanent Resident Card as not doing so may confuse border officers which might conclude that you are traveling to Canada as a visitor and not as a permanent resident. In some cases, officers could issue a removal order and put your permanent residency at risk. You should always have the documentation related to your status when traveling.
Here are some other documents you may find as a useful resource.