Ahimsa literally nonviolence. It may ring a bell because it was the famous codeward of Mahatma Gandhi's movement resistance to the British occupation of India. Ahimsa means not hurting yourself.
There are some situation where causing harm cannot be avoided. In the well-known example from the Mahabhrata, Krishna advises Arjuna to take up arms against a prince whose evil actions must be stopped. Note that Krishna makes this suggestion only after multiple attempts at peaceful negotiation have failed.
Hindus are generally a peace-loving people. Yet only a minority go so far to say that even in cases of self-defense or protection of the innocent, violence must always be avoided. Hindus generally acknowledge that the ahimsa movement worked because Mahatma Gandhi was defying the British. If he had been up against the Nazis, the outcome might have been different.
Nevertheless, practicing Hindus takes ahimsa quite seriously. A Canadian friend of mine was mentioning how each time he would leave the home of a Hindu friend she would hand him a sweet and say to him", If I have offended you in any way during your stay in my home, please forgive me". This was a women of extraordinary kindness who he couldn't even imagine anything offensive! Yet it was important to her to ensure that if she had inadvertently anyone, she is clear at once.