Dharma is a Sanskrit word originally meaning order or solidity or normality. It comes from an earlier Indo-European (Yamnaya) word meaning firm, solid, like wood or a tree. Hindus later on used it to mean something like “law” or “duty” – it’s the laws of nature that keep order in the universe, like Greek “nomos“. But, like Greek “nomos”, dharma is also the laws of the gods and of people, that tell us how to behave, upright like a tree.
People sometimes think of dharma as being related to the Chinese idea of the Tao – the Way, that is the right way. The more you live in accordance with your dharma, the quicker you stop being reborn and get to nirvana.
In Hindu thought, people had different dharma depending on who they were. People were supposed to know their place and behave according to whether they were boys or girls, their caste and their stage in life – children had different dharma from their parents, for example. So the idea of dharma was one of the main ways of convincing people to put up with the oppression of women and the oppression of the caste system and slavery. But it was also a way for people to feel that they knew who they were and what they should do – for people to feel centered in the world.
The Bhagavad Gita is a section of the Indian epic The Mahabharata where the god Krishna explains dharma to the prince Arjuna. Arjuna doesn’t want to fight and kill people in a battle, but Krishna tells him that it is his dharma, and finally Arjuna agrees to fight.