The acronym PRB stands for post-retirement benefit. A person receives a post-retirement benefit when they are receiving Canada Pension Plan (CPP) payments and they are still earning employment income. Also known as retired and still working. As this person is receiving CPP and still paying into CPP because they are employed, there is an additional benefit calculated, over and above their original CPP entitlement.
Prior to 2012, a person who was working and had already started taking their CPP did not pay into CPP. After this rule was changed the PRB was born. The PRB is a payment received for pension earned while working, after you have started to receive your CPP.
If you are between the ages of 60 and 65, earning employment income and receiving your CPP, you will be earning a PRB. After you turn 66, if you are still working you can elect to stop paying into CPP. Once you are 71 you are no longer permitted to pay into CPP. Once you stop paying into CPP your PRB will not grow any more, but you will receive that payment and your CPP every month for the rest of your life.