We can generate a 'hash' of a file which is basically a big number that will generally be different for every file. It is very hard to create two files that will have the same hash. It's often used to prove that a file hasn't been changed (accidentally or maliciously). There are various algorithms to do this. SHA-1 has been around since 1995 and is considered broken.
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So, it essentially is an outdated security feature which is prone to hacking?
Yes. Clever people find ways to shortcut breaking it. Google and others have been encouraging sites to upgrade to something better, but some of them move very slowly. Changing how your site works is likely to break it, so they are very cautious.