Dharmas are the ultimate elements or particles of the universe . A little like atoms, they are very small, but they exist for only a split second, in keeping with the doctrine of impermanence. And while atoms are purely material, dharmas include all phenomena, mental and physical. I like to think of them as little flashes of colored light, and I would translate the word as scintilla. Don't get confused between these and the Dharma, meaning the teachings of the Buddha!
Like the ancient Greeks, the ancient Buddhists thought there were four basic elements: earth, water, air, and fire. The dharma theory turns these elements into qualities, or even verbs: fire becomes hot becomes burning; air becomes cool becomes blowing.... Ultimately, then, all things are nothing more than bundles of these qualities or actions, and are empty inside. This led to one of the most important ideas of the Madhyamaka School of Mahayana Buddhism: Shunyata, which means emptiness.
In Mahayana Buddhism, the dharmas were considered something more like phenomena than atoms, and the Yogachara School took the change even further, and considered them something more like ideas in the universal mind.