I know its only been a couple of hours since I asked but I am itching to upgrade!! My google Fu is failing me
EDIT: I mean of the zip file.
Every file has a unique 'code' (as such). You can verify the file is complete/as it is supposed to be by comparing the checksum of the original file against the copy you have.
Simplified:
Person A creates Zip File X, It Generates a checksum of ABCD.
Person B downloads Zip File X, Runs a checksum tool to check the checksum of the file, it returns ABCD - The file is complete and identical to the one you should have.
If it returns a checksum of EFGH - the file is incomplete/corrupt/not the same.
It's important to verify the checksums match as flashing a phone with a corrupt/incomplete/invalid file can potentially brick it.