This is the key sentence of the Heart Sutra. By the light of the prajna of contemplation, it illuminates that the Five Skandhas are empty. We can see matter, yet it is fundamentally empty. Matter's own nature is emptiness. In Buddhism, all matter is formed by Four Great Elements (i.e. earth, water, fire, and wind) under certain causes and conditions. Just like our body, the Four Great Elements unite to become a body, and when they separate the body is destroyed. Each of the four elements restores its original state, which is emptiness. Most people are attached to the body as "me", which is one of the greatest ignorances of people.
The same applies to sensation, recognition, volition and consciousness.