NZB.is is a Usenet (newsgroup) indexing site. It does not provide any files itself, other than NZB files. The way it works is that the NZB files you download from NZB.is contain a list of pointers to the real files on the Usenet network. So, NZB.is provides one of the three things that you need to download files: the index. You will also need a downloading program, like Super-NZB, and a Usenet server like Easynews.
In the screenshot below, we see a search done for the freeware Ubuntu operating system. To download the first file in the list, click the NZB icon (red circle) and save the file to your computer. Next, you open that file with Super-NZB, and once you enter in the Easynews URL, Super-NZB can download the file. Click picture to enlarge: