Clicking this will display a browse window where you can choose the image file(s) you want to add to the list of files to be resized. If you want to choose more than one file from a given directory hold down the control key while clicking on the file names. If you want to select all the files in a directory, click on the first file; hold the shift key; and hit the end key. Once you have highlighted all the file(s) in the current directory you want to resize click Open. If you want to resize files from more than one drive or directory you will need to click this button more than once.