"Content first" is a way of developing websites where you literally start with the content before you start the design. With Content first, you focus on the content and the structure of your site before anything else, leading to a more logical structure and layout.
In an ideal world, every project would have the entire content mapped out before the start of the design and before the start of development. Practically speaking though, this is almost never the case (and there is a point to be made about the content being created in tandem with the rest of the site). However, having some content ready will greatly improve your design, and having an idea of which content you want to write (for example, by creating a sitemap) will already improve your site's structure.
