Exercise is really important for puppies, for their health and also for their socialisation, lead training, and mental stimulation. Getting exercise every day should be part of the routine for all puppies, but this does need to be controlled.
While puppies are young they are still developing and growing. In order to allow for this, the ends of the bones have growth-plates which allow bones to get longer. When pups are fully grown these growth plates harden up into bone themselves, and your dog has reached their adult size!
Normal exercise helps this process, but while the bones are still growing excessive exercise or impacts can injure or affect the bones or growth plates. For this reason, it is important that puppies are only exercised within their limits until they are fully grown.
While most puppies are fully grown by a year old, some giant dog breeds mature much more slowly, and can take 18 months or more for their growth plates to fully close.