You have probably seen these below the rom or file you are downloading: MD5: 4576d484f39792ab0cfb506ea0592150
So what is MD5? MD5 is a number used to make sure two files (yours vs theirs) are the exact same using a mathematical equation. The internet sends everything in bits and parts. Sometimes things get lost along the way downloading. MD5 is used to make sure the files match. A file can be corrupted any time it is moved (or even while being on your hard disk if it's bad). This includes moving from your computer to the phone, since USB is far from perfect.