Students have to undergo a medical exam, after which they will be issued with a Medical Certificate. The Medical Certificate, in turn, is required before they can be issued with their Student Pilot Permit, which enables them to fly solo as pilot under the supervision of a Flight Instructor. You can start your training before getting but cannot fly solo without it.
The medical requirements are established by Transport Canada, and are basically designed to ensure that a pilot is not at risk of a medical condition that could incapacitate them during flight. Private Pilot must qualify for a Category 1 or 3 Medical Certificate, while Commercial Pilots must qualify for Category 1 Medical Certificate. See Transport Canada Medical Requirements