This is a broad aim of not causing pain, loss, or destruction to any living thing. At a minimum, this is a sweeping resolution to avoid any whit of harm to another human being. The implications are far-reaching, since most of us participate daily in activities whose requirements or ripples may involve harm to others (e.g., use of fossil fuels that warms the planet, purchasing goods manufactured in oppressive conditions).
Further, in American culture there is a strong tradition of rugged individualism in which as long as you are not egregiously forceful or deceitful, “let the buyer beware” on the other side of daily transactions. But if your aim is preventing any harm, then the other person’s free consent does not remove your responsibility.
Taking it a step further, to many, harmlessness means not killing bothersome insects, rodents, etc. Even as you feel the mosquito sticking its needle into your neck. And to many, harmlessness means eating a vegetarian diet (and perhaps forgoing milk products, since cows need to have calves to keep their milk production flowing, and half of those calves are male, who will eventually be slaughtered for food).
Nonetheless, we need to realize that there is no way to avoid all harms to other beings that flow inexorably through our life. If we are to eat, we must kill plants, and billions of bacteria die each day as we pass wastes out of our bodies. If we get hired for a job, that means another person will not be.
But what we can do is to have a sincere aspiration toward harmlessness, and to reduce our harms to an absolute minimum. And that makes all the difference in the world.