Code of Conduct
A Code of Conduct is a document with a set of rules one is required to follow in order to participate in a free software project, a group, a conference and many other social interactions.
The vast majority of large corporations have long had a strict set of they require their employees to follow. This isn't the case if you and your friends meet up at one of your friends houses, there's no corporate control there. The same is to some degree true when it comes to free software projects.
Most free software projects have some, and in many cases a majority of, people who are employed by large multi-national corporations. Those who "participate" in free software projects are required to follow corporate rules or they will get fired. Developers who do voluntary work in their spare time can't be fired if they don't follow the internal rules of a single corporation.
This is why the "Code of Conduct" cancer was invented: It's simply a way of imposing corporate rules on non-corporate citizens. It really is that simple.