Support on Goodpoint simply refers how an idea is justified. In other words, it's what the writer adds to make the idea valid and persuasive to readers.
If the idea 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 article that supports your idea, whether a fact or an opinion.
The Goodpoint editor interface allows you to support an idea in these ways: by adding more ideas or by simply expanding your idea into several paragraphs of writing.