What is content modelling? Rachel Lovinger says:
A content model is a powerful tool for fostering communication and aligning efforts between UX design, editorial, and technical resources on a project. By clearly defining the assembly model, the content types, and the content attributes, we can help make sure that the envisioned content strategy becomes a reality for the content creators.For me, in essence, it's about thinking things through enough to let your content live beyond the web page it was initially envisioned for.
Rachel has written a great article which takes us through why content modelling is an important aspect of content strategy and how you go about doing it.
Content Modelling: A Master Skill - article by Rachel Lovinger for A List Apart
Another excellent resource is Cleve Gibbon's content modelling series. Here, Cleve outlines the role of content modelling as a effective communication tool for structuring content, and provides some great further reading and a glossary.
And you really can't get into content modelling without beginning to think about metadata. Or structural metadata at least. If you've never thought about the different types of metadata, and why each one matters you would do worse than to check out Rachel Lovinger's slides: