Fitness is a basic skill that indirectly indicates how athletic Sims are.

 Fitness in a vacuum does not affect a Sim's weight; a Sim with Level 10 Fitness can still be obese depending on how long it's been since they've worked out. The only aspect of the game Fitness directly impacts is the chance of the Sim winning a fight.

Increasing levels of Fitness is as simple as working out. Every neighborhood has a public gym that a Sim can visit for free, or they can use a self-interaction to "Go Jogging" no matter where they are. Once a Sim has some cash, they can purchase workout equipment in Build Mode for their own home use.

After a Sim works out regardless of where they do so, they will begin to tire and automatically stop on their own. You can order them to continue with a "Push the Limits" command, but it's highly likely they will then draw a "Sore" negative moodlet and become uncomfortable for several hours. It's best to allow them to stop when they're tired and, if you really need to power-level Fitness from there, to order them to read a Fitness skill book while they rest. Note that if you have the Spa Day Game Pack installed, you can get a massage to erase their sore muscles.

As Fitness increases, Sims can workout for longer periods of time, and the Sim's workout options also increase. A Sim can perform No Sweat workouts to remove the Hygeine meter penalty, or Endurance workouts to go longer before they get too tired or sore. If you own the Fitness Stuff Pack, you'll have additional options as well, such as a rock climbing wall.