They include grains and products made from them. This means wheat, corn, oats, rice, etc. Flour, bread, and other baked products.
Grains are excellent source of complex carbohydrates, which give you steady and reliable source of energy. They are also good source of plant protein. This protein is incomplete though, and you have to eat other plant sources, or animal products, to get all the amino acids that you need in adequate quantities. Good combination for getting complete plant protein is grains + legumes (for example rice and peas)
You should preferably eat WHOLE grains instead of refined grains. Try to avoid white flour, white bread and white rice. Whole grains have more fiber, which slows down absorption of carbohydrates, and in such a way gives a better source of energy, prevents blood sugar spikes, and is good in prevention of diabetes. Fiber also helps with digestion.
By supplying high fiber content, whole grains lower the risk of several diseases, including coronary heart disease, stroke, cancer and type 2 diabetes, and are associeted with lower all-cause mortality. Regular whole-grain consumption lowers LDL and triglyceride levels, which contributes to reduction in coronary heart disease-risk factors. In addition, whole-grain consumption is inversely related to hypertension, diabetes, and obesity when compared to refined grains, all of which are negative indicators in total cardiovascular health.
Whole grains also have much more vitamins and minerals, because they are mainly concentrated in bran.
You can base your diet on whole grains, eat them often, but still try not to eat too much because they still contain carbohydrates and calories, and if you eat excessive quantities you can still gain weight.
Note that grains generally require either boiling or baking. They are not supposed to be eaten raw!
Commercial breakfast cereals are not so good, they are processed, they have additives, etc. Instead, you can buy raw whole grains, (oats are among the best), soak them in the water over night and then cook them for about an hour. This makes an excellent and very nutritious breakfast.
You can also eat bread made of whole grain flour. Bread is an important part of the diet, but whole grain bread is much better due to fiber and vitamin content. Bread is a staple food recognized by cultures and religions, and therefore there’s no reason to avoid it. It’s important to eat bread in moderation though.
Avoid puffed grains (for example puffed rice and popcorn). Puffing is an artificial process which causes the grains not only to lose some of their nutrients, but also it increases the speed of absorption of sugar immensely and causes extreme sugar spikes, and subsequent insulin shocks.