A checksum is the outcome of running an algorithm, called a cryptographic hash function, on a piece of data, usually a single file.
So you have a single file, giving into an hash generator you will have your checksum.
By comparing the checksum that you generate from your version of the file, with the one provided by the source of the file, helps ensure that your copy of the file is genuine and error free.
That’s the way we prevent hackers to inject some code into downloaded files.
A Checksum example
First of all, take a look at the picture below
You can see, we have a string with the text “Abc”.
Now take a look at this MD5 Hash:
You can see the difference between Abc and abc hash string.