As soon as a task is opened, Flyspray starts logging changes to that task in the History. If you are a member of a group with the View History permission, you can view the History Tab by clicking the History heading.
There are a wide variety of things logged to the history, including all changed fields, comments added and edited, files attached, notifications list changes etc. The history keeps track of old and new fields, and the person who made the changes. Keeping track of all these things allows for comprehensive reports to be generated by Project Managers.