Normally, you would place the cursor on a star and press X or x. The column and row profiles through the center of the cursor are displayed along with a measure of the FWHM (full width at half maximum). The FWHM is a measure of your seeing and can be used by the Deconvolution function to sharpen the star images.
Only the data in the box is shown and it is adjusted for the background threshold, i.e. the background threshold is subtracted from the actual data before the difference is displayed.