Natural laws are:
a principle or body of laws considered as derived from nature, right reason, or religion and as ethically binding in human society.
Morally and philosophically speaking they are laws that we agree on as a society as part of a moral and ethical code of conduct. This intersects with the legal interpretation where 'code of conduct' strengthens and becomes 'law (of the land)'. By moral justification, if legal natural laws are broken then it is socially agreed that the wheels of justice be set in motion.
In Greek philosophy, Aristotle distinguished between natural law and legal law (or common law) by noting that natural laws were universally true whereas legal laws varied from place to place (jurisdictions and cultures). This leads to the idea that all individuals have basic human rights. These rights can be protected by international law.
Philosophers who have theorized about natural law include for example Thomas Aquinas, Alberico Gentili, Francisco Suárez, Richard Hooker, Thomas Hobbes, Hugo Grotius, Samuel von Pufendorf, John Locke, Francis Hutcheson, Jean Jacques Burlamaqui, Emmerich de Vattel, Cesare Beccaria and Francesco Mario Pagano.
Natural law can also describe a law observable in nature, for example gravity and other laws of motion, and planetary motion--that is, physical laws. This of course assumes that only physical things can be observed, via scientific measurement. Observation that cannot be proven scientifically and is not repeatable in experiments is not, in scientific thinking, a natural law.