"A dharma," the Buddha said, "is whatever ordinary people and followers of lesser and heterodox paths imagine. Basically, they think a dharma has existence and substance and arises from causes. Such things must be abandoned and avoided. Don't engage in the projection of appearances or become attached to what are perceptions of your own mind. The things people grasp, such as clay pots, lack any real substance. To view dharmas like this is to abandon them,"
Let's not be unclear about anything. How can a dharma, a real thing, not have existence and substance? Why should it be abandoned?
To understand, first remember Animal Crackers. Animal Crackers were little cookies cut into the shape of lions and bears and elephants and so on, but they may as well have been cut into the shape of people and airplanes and mountains, too, because they were all just one substance: cookie dough. The lions, the bears, the elephants, the people, the airplanes, and the mountains were all nothing but cookie dough, but they were cut from the dough into the different shapes and forms.
All of the universe is of one substances, formless and undifferentiated, but the mind is like the cookie cutter that separates things into animals and different people and tea pots and so on. But without the cookie cutter of the mind, all things are one substance, interconnected and undifferentiated.
The Buddha is telling us that all things are devoid of separate existence and any real substance because they are only our mind's cookie-cutter differentiation into various shapes and forms. But they are just perceptions of our mind, how we choose, sometimes arbitrarily, to segregate little parts of the cookie dough from the whole undifferentiated fabric of the universe. Another mind might cookie-cutter the same thing differently, and what I differentiate as "a friend" that other mind might differentiate "an enemy."
This, of course, is how we live and survive. If we don't differentiate the oncoming bus from the cookie dough around us, we won't live very long. But the problem, as Red Pine pointed out, is not the dharmas themselves, those Animal Crackers of the mind, but attaching to the concept that those things are separate and real and not a part of the cookie dough, that they are anything less than the Animal Crackers of the mind. This becomes really significant when we realize that those Animal Crackers are not just the external world, but that our concept of "self" is just another one of those Animal Crackers. We only exist separate from the dough because we've cookie-cuttered ourselves from it.
"Names and forms are made by your own thinking," Korean Zen Master Seung Sahn once said. "If you are not thinking and have no attachment to name and form, then all substance is one." Meditation is that state where we quiet our thoughts and are consciously aware of what is beyond thinking. It is our opportunity to experience the cookie dough.