Catabolism, in brief, is the breakdown of large molecules into smaller ones. In the context of metabolism, these smaller substrates are things such as TCA Cycle intermediates/products or ketones from fat/ketogenic amino acids.
When in ketosis, the body begins to burn stored fats for energy, and this results in the build-up of acids, known as ketones, in your blood. A person following the keto diet aims to burn unwanted fat by actually pushing the body to rely on fat, not carbohydrates, for energy.
For better understanding, the student must watch the following youtube videos