In his book Sapiens, Yuval Noah Harari makes this clearer than I did. He refers to it as intersubjectivity: a shared agreement of meaning between multiple people. Laws, and the fact that money has value, are intersubjective truths, defined by society.
Gravity has — more or less — been a constant, physical truth. Slavery is abhorrent today, but 1,000 years ago was part of life — hell, even 200 years ago. Truths change as our society evolves and our morals progress. They come from a present day context and are enforced by people rather than nature. This doesn’t diminish their importance or weight, if anything, it is even more important that we uphold them.