A list of all the content on your site, the: copy, imagery, links, keywords, word counts, page length, metrics, etc. A content inventory is a tool for you, your team, and your stakeholders to understand the nature, depth & breadth of your content, and any work it needs (such as rewrites or creation), including the origin and destination of said content during a project or product lifecycle.
When to Use It
At the beginning of a project, or product lifecycle, you need to understand how much content there is, what shape it is in, and what the plan is for it. Complete a content inventory before decisions about technology and design are written in stone. Technology and design should be fitted to the content required for people to do their work, not the other way around.