It is flavor text that means that your ConstitutionConstitution score has gone up by one point. Usually because a number of things such as using Constitution-based skillsConstitution-based skills or eating certain kinds of foodsfoods. Of note, Medium Armor and Heavy Armor are CON skills that train simply by wearing that armor and moving around, while Polearms and Staves and Blunt all train just by attacking. But perhaps more to point - Healing, which is your natural recovery, trains any time you heal up. So Constitution tends to train pretty well in the background as a result, for any character.