An idea is a kind of content that you can vote on. When you add an idea, others in the community will be able to promote it by voting up or demote it by voting down. If they vote up, your idea gets more points; if the vote down, points are taken away.
Creating an idea is a lot like creating a document. You give your idea a title and type its content. As with documents, you can format the idea's content as you would apply formatting in a word processing program. For more on creating ideas, see How do I add an idea?
As with other lists of content, you can view lists of ideas, such as by clicking the Ideas tab in a space overview.