Tone can also be called 'thickness' or 'muscle tone'. It's basically how fit you
look, independent from how strong your character actually is, and there are two separate stats. One is how fat you are, and the other is how much muscle definition your character has. There's a few things that can shift it up and down, you can generally find out what in the CoC wiki but its not 100% obvious sometimes. For example,
LaBova increases thickness, and decreases tone. Meanwhile, a
Fox Jewel exclusively decreases muscle tone, without increasing thickness. Off the top of my head, I believe there are several foods you can buy in the Tel'Adre bakery to increase thickness, and a specific training type in the Gym increases muscle tone. Hope that helps you find the stuff you need.