How are the validity of moral obligations objective?

I’m currently studying ethics and relativism is a subject we are currently reading about. I want to understand, how are obligations, which are moral or legal conditions that depend on accepted terms and beliefs in order to be committed, objective in contrast to subjective? What evidence or inference has been or is in existence, which states that obligations are objectively valid? In other words, are obligations, which are moral by definition, humanistic or independent thereof?