Because of restrictions amongst Usenet servers, articles containing a file can often be a maximum of 50 Megabytes. Binary articles bigger than 50 MB are therefore broken onto pieces so they can be distributed on Usenet while obeying the limits. Your client downloads al lthe different pieces and 'glues' them together, if you will. But sometimes, your client can't find all the different pieces it needs because they got corrupted or are lost. This is what parity files are for. They are bits and pieces of data that enable your client to fill in the blanks of another missing piece of data it needs to successfully rebuild your download. Most modern clients will download, repair, rebuild and extract your download without a hassle. But sometimes you need an extra software package to do the rebuilding for you.