Support on Goodpoint simply refers how an argument is justified. In other words, it's what the writer adds to make the argument valid and persuasive to readers.
If the viewpoint is a fact, it is the information that the writer thinks makes it true. If it is an opinion, it is why the writer thinks the viewpoint should be shared by others. The third way is to add a URL of an existing post that supports your argument, whether a fact or an opinion.
The Goodpoint editor interface allows you to support an argument in these ways: by adding more viewpoints or by simply expanding your argument into several paragraphs of writing.