I'm not sure what's not to understand here, it all appears fairly obvious (to
me at least). Anyway, here's a definition...
Project Statistics
A project-level summary displaying a comparison between baseline, current,
actual and remaining values for the project's schedule and its duration, work
and cost. Also displays percent complete and percent work complete values for
the project as a whole. Accessed by the Project Statistics button or the
Project..Project Information..Statistics command. This information can also
be printed by using the 'Project Summary' report.
And here's the information from the help file
Statistics Opens the Project Statistics dialog box for the current project,
which you can use to review scheduled (schedule: The timing and sequence of
tasks within a project. A schedule consists mainly of tasks, task
dependencies, durations, constraints, and time-oriented project information.)
and actual (actual: Information that shows what has actually occurred. For
example, the actual start date for a task is the day that the task actually
started.) start and finish dates, durations, work, and costs.
I very much suspect that is all the documentation available.