dharma is righteousness and a-dharma is that which is against dharma. in a very broad sense it is good and evil respectively. just as an example, in the epic mahabharata, the kauravas snatching away the kingdom of the pandavas (first by dhritrashtra due to his greed/ ambition and later by duryodhana due to his greed/ jealousy) is a-dharma. on the other hand, yudhishtir treating dhritrashtra and gandhari as his own parents after the end of the war (forgetting the many ways dhritrashtra and his sons had wronged them innumerable times) is dharma.