A pilot proficiency check is a demonstration that you are competent to do the job assigned to you as a flight crew member. It is a requirement for flying a multi-engined aircraft, turbine helicopter, and certain high performance single engine aircraft such as the PC-12.
Think of it as the flight test for flight on instruments, and the check-ride required for a multi-engine rating, rolled into one check, with the items specific to that particular type included.
This PPC ride is required annually for aircraft in Canada under 12,500 pounds, and every six months for aircraft over 12, 500.