Keto (rhymes with Cheeto, but otherwise has nothing to do with it) is short for ketogenic. Eating a diet that’s ketogenic means that your body has shifted to a state of ketosis where you’re burning ketones for energy.
In general, a Ketogenic diet is framed as a high-fat/moderate protein/low carb way of eating. Sounds familiar, doesn’t it? That’s my general approach in The Wonderfulness Program. It’s the basic way to break your body’s dependence on glucose for energy so it can begin using fat (in particular, your own stores of body fat) instead.