Grass is a common term with various colloquial forms.
Proper Definition: Any of a large family (Gramineae or Poaceae) of monocotyledonous plants having narrow leaves, hollow stems, and clusters of very small, usually wind-pollinated flowers. Grasses include many varieties of plants grown for food, fodder, and ground cover. Wheat, maize, sugar cane, and bamboo are grasses. Source: Dictionary.com.
Grasses usually have the same general structure. Plants in the grass family have narrow leaves with parallel veins. Grass leaves are called blades and they attach at the nodes. The leaves wrap around the culm before they start to stick out. The part that wraps around the culm is called the sheath and the part that sticks out is called the blade. Grasses have flowers that grow in a structure called a spikelet. The flowers are pollinated by the wind. Once the flowers are pollinated, the seeds form. The seeds are dispersed by the wind, rain, and sometimes by passing animals.
The seeds are what contain the most digestible nutrients of the plant for human digestion. To access those nutrients the seeds need to separated from the chaff surrounding the seed. From there you can cook the seeds like a whole grain (such as brown rice) or mill it down into flour. This part of the plant contains carbohydrates that can be absorbed and turned into sugars that help power cellular function.
The chaff, sheath or hull of the grain as well as the blades of grass are primarily composed of cellulose. This is integral in the plants formation because they are the photosynthetic cells that the plant uses to convert light into sugar energy for its own growth. Cellulose is a fibrous material with limited nutritional value for humans.
For example, corn is within the grass family. When you eat a corn cob, your digestive system can only break down what on the inside of the kernel. So when the corn has passed through your system, you will often see yellow husks in your excrement. These are the indigestible cellulose shells. Those shells are also the hard bits that get stuck in your teeth in partially popped pop corn.
This would occur similarly if you were to consume blades of grass, even if it was milled or boiled to soften it. These cellulose fibers are also not water soluble, so other than the imparting a green color caused by exploded chloroplasts, boiling does not actually have any nutrients dissolved in it. Water is also a critical vehicle in the absorption process, so if it is insoluble in water, it’s unlikely to be absorbed in the small intestine. For a nutritional analysis of this theory, I suggest reviewing the dietary and nutrition of eating Celery stalks.
However, bear in mind that vegetarianism is different from being a grazing herbivore. There lots of other plants that are not grasses, and aspects of plants that are readily consumable for humans. Fruit (apples, tomatoes, etc), vegetables, berries, legumes (nuts), fungi, and starchy roots (potatoes, yukka, etc.) are all bioavailable vegetarian nutrients sources for humans, with precautions taken for potentially toxic varieties.