MD5 is a checksum that verifies the integrity of a zip or rar or .7zip file. When you make the .zip, .rar, or .7zip file, you make a MD5 number off of that file, or a HASH file, hash code, MD5 code, whatever you want to name it. Anyway, it uses an algorithm to give you this numerical/alphabetical code which you can then compare to your file.
WHY? Well you are transferring this file from device to device, or through the pc, through the internet, through a server, and things can get lost or stolen. Antiviruses can steal pieces, servers, or data loss through the internet can lose pieces, yet the file SEEMS the right size, and can SEEM like it is ok, and in fact, you flash it to your phone and things do not work.
This is a reason I like torrents. They make, and check an MD5 for you, and remove some of the risk of a bad download, however that is for a different guide.
In this guide I use the free, and easy MD5 checker you can get from the play store. I am crazy so I check my md5 on my pc, then again after the transfer to the phone. You may just check it once on the phone, to save time.
Download the file to your phone. Copy the MD5 alpha/numeric. for the guide I had transfered the file from my computer, so I made a .txt file to save the md5 in. The screen shots pretty much speak for themselves... Keep in mind that I was using a beta rom you will have your rom and your rom names. I used root explorer, rb text editor, and MD5checker. Many thanks to those who provided these tools, and this rom I was using in the guide.