Harm as related to stealing/theft is the basic position to start with. Don't steal life, sexual preference, bodily integrity, property. Actions are objective, they happen. Their effects upon others can be objectively discerned.

A codified law by an authority can confirm moral law, but can also go against it. Writing it down is a way to convey it to others who haven't figured it out ;) A way to explain it to them ;)

The term morality, moral law, moral truth, get obfuscated, because societies make "moral codes" so to speak, and laws, based not on moral truth, but on whims.

Objective morality is when harms is incurred. Then there are nuances, like a doctor harming to save, or speaking the truth that can hurt, or self-defense or stopping an evil person with "harm"/"aggressive due force not violence. Unjustified harm is immoral.