The Forum activity reports list all posts an individual student has made in forums within the course site. This aggregated list of posts gives you an efficient way to review the student's contributions.
In your course site enter a forum
Click on a student's linked name to enter that student's Profile page
On the student's Profile page, in the Miscellaneous section:
Click on Forum discussions to see all topics the student has started
Click on Forum posts to see all individual postings the student has made in all forums in the course