The phrase "return to full fitness" refers to regaining one's health and strength after an illness, injury or surgery. It means being able to perform physical activities at the same level as before the incident occurred. A person may have lost their ability to exercise due to a medical condition, but with proper treatment and rehabilitation they can work towards getting back to where they were physically before the issue arose. The process of returning to full fitness involves a gradual increase in activity levels, under the guidance of a healthcare professional, until one is able to resume their normal routine without any limitations or restrictions.