These are applications that you can download on to a USB drive or memory card and use anywhere. It's a way to have the software you like available wherever you are. There's a large range available, including image editors, accessibility tools, video viewers, DVD burners and more.
Illustrated on the right are some particularly useful: Opera, Chrome and Firefox browsers, Komposer web design (similar to Dreamweaver and FrontPage) and Gimp for design work.
The link goes to the PortableApps site from where you can easily track down what you need.