An framework to move content through a defined set or map of specific steps, activities, or tasks.
The content can only be "in" one such state for a particular workflow at a time. When that state resolves, content will "move" to another state, and continue until the workflow ends.
What constitutes a workflow step is highly configurable. Workflow is often used to approve content, though many CMS have separate approval systems, or allow the functional equivalent through permissions. Other activites might change the content in some way, or execute arbitrary code.