On the standard keto diet, you plan all meals and snacks around fat like avocados, butter, ghee, fatty fish and meats, olives and olive oil. You need to get about 150 grams a day of fat (the amount in nearly ¾ cup of olive oil and three times what you are likely eating now) in order to shift your metabolism so it burns fat as fuel. At the same time, you need to slash your carbs from about 300+ grams per day to no more than 50 (which is about the amount found in just one blueberry muffin). That means sticking to leafy greens, non-starchy veggies, and low-carb fruits like berries and melon. Finally, you'll eat a moderate about of protein, which is about 90 grams per day or 30 grams at each meal (think 4 ounces of meat, fish, or poultry).
One of the boldest claims about the keto diet is that it can help combat cancer by effectively "starving" cancer cells. It's a claim based on a phenomenon dubbed the "Warburg effect", which, in simple terms, describes how cancer cells are more reliant on sugar than healthy cells. So, the idea is that sticking to a low-carbohydrate diet will make it harder for cancer cells to thrive.
At the core of the classic keto diet is severely restricting intake of all or most foods with sugar and starch (carbohydrates). These foods are broken down into sugar (insulin and glucose) in our blood once we eat them, and if these levels become too high, extra calories are much more easily stored as body fat and results in unwanted weight gain. However, when glucose levels are cut off due to low-carb intake, the body starts to burn fat instead and produces ketones that can be measured in the blood (using urine strips, for example).