High Metabolism is an Aspiration Trait. All aspirations give your sim an extra trait so that they can complete their aspiration easier. Mostly this aspiration trait will give your sim certain moods or desires when performing a task related to the aspiration Athletic.
Fitness is very important in the world of the Sims 4, because there's not always a carpool waiting for your sim. Some sims choose to also totally ignore the fact that they can teleport to their work, but choose to run there instead. In these cases the High Metabolism trait can help sims stay in shape so that they can do their daily tasks.
Sims that have this trait do not need to worry about what they consume anymore. They could eat Hamburger Cakes, fast food and waffles without gaining some extra pounds.