Mahayana means the great vehicle, it is the Buddhism of China, Tibet, Japan and Korea. It arose around the first or second century CE as a reaction against several highly analytical schools of Buddhism which had developed in the 600 years since the time of the Buddha. These schools were referred to as Hinayana, the lesser vehicle by the Mahayanists. Zen, which appeared around 800, in China is considered a school of Mahayana.
Avalokitesvara Bodhisattva, When practicing deeply the Prajna-paramita perceives that all five skandhas are empty and is saved from all suffering and distress.
Avalokitesvara is an enlightened being, a Bodhisattva, who has forgone his own entry into Nirvana so that he can help others. As the embodiment of compassion he is called on by traditional practitioners of Buddhism in times of crisis. In the Heart Sutra he has realized emptiness through the practice of Prajna-paramita (infinite wisdom) and is preacing to the monk Shariputra.
That which is form is emptiness that which is emptiness form. What is Emptiness? Emptiness is how we translate the Sanskrit noun Sunyata. The adjective form is Sunya, Empty.
Does Emptiness mean that Buddhists believe that nothing exists? No, Emptiness is not nothingness. It is the other side of interdependence (pratityasamutpada). All things are interrelated, you cannot take out an object and say this is here in and of itself. Its existence has no self-being (svabhava). This is explained further by Avalokitesvara using the five skandhas. The same is true of feelings, form, perceptions, impulses, consciousness.
These are the five skandhas (aggregates): form, feelings, perceptions, impulses, consciousness. It is how we are aware.
Form is the solid object, the color or the sound that is interdependent with the other skandhas. For example a hot stove.
Feeling is the act of the sense organ contacting the object. The skin feels, the eye sees, the ears hear. For example when the skin comes in contact with a hot stove.
Perception is the first sensational awareness of the object. For example the skin becomes hot when it touches the stove.
Impulse is the unconscious reaction to the object. We place our hand on a hot stove and our impulse is to pull it away, before we even think.
Consciousness is mental awareness of the object. One feels the heat on the hand and thinks, "Ouch!"
Each of the skandhas is empty, since it cannot exist on its own, it is dependent on the four other skandhas. Like a set of blocks forming a house, the entire structure is dependent on its pieces, you can't point to one block and say that block alone is the house.