Portfolio is list of your works (screenshots) - it can be screens of things you have written, designed, developed, proofread, SEO achievements, anything that proves you know how to do the work.
improves the chances of getting hired! This will be displayed as gallery + files, see here
for example (opens new window). You can upload here example images of the work you've previously done alond with any PDFs, DOCs that explain your work in detail