Diet means the usual food and drink consumed by a person or animal. A healthy diet for your pet is not the same as your diet, so you shouldn't feed your dog all your table scraps.
If you limit the number of calories you eat in order to lose weight, or restrict certain foods for your health, you can also call this a diet. If you want to lose 10 pounds by summer, you might have to go on a diet. In certain countries, the word diet, though pronounced the same, has a non-food related meaning: the diet is the legislative branch of the government in countries like Japan.