To make it very simple: It is an agreement between two people or groups that is spoken not written. The only way to keep it together is to stay in touch. Over time a spoken agreement can be forgotten even if people didn’t mean to. To stay in touch means to speak about it again. If you don’t “keep it alive” in memories of all involved it will fade away.
The human race has learned to rely more on written agreements so that people have the documents to stay in touch with as reminders. The documents allow more complicated agreements. Written agreements that are signed and witnessed help people outside of the agreements(such as helpers, assistants, other contacts) understand what is going on, including law professionals and courts of law.