A "Portable" app is a version of software designed to run without being installed on a host operating system. Ideally, you could carry it on a USB drive, plug it into any PC, and start drafting.
If you're looking for a free or affordable CAD solution, you might consider: Autodesk Autocad 2013 Portable Torrent
Would you like more information on CAD software or alternatives? A "Portable" app is a version of software
While it may seem appealing to save money by downloading software via torrents, there are substantial benefits to obtaining software through legitimate channels: plug it into any PC