A worksheet is the BUI screen on which statistics are graphed. Multiple statistics can be plotted at the same time, and worksheets may be assigned a title and saved for future viewing. The act of saving a worksheet will automatically execute the archive action on all open statistics - meaning whatever statistics were open, will continue to be read and archived forever.
See the Open Worksheets for how to drive worksheets, and the Saved Worksheets for how to manage previously saved worksheets.