The document provides an introduction to cumulative flow diagrams (CFDs), which are a visualization tool used in Lean/Agile software development to track the status of work items over time. It explains that CFDs show work items moving through different workflow states like "to do", "in progress", and "done" to provide insight into metrics like throughput, work in progress, and cycle times. Examples of CFDs are shown tracking work items in a Kanban system over the course of a day and for a Scrum team over multiple sprints.