it's pretty easy to explain, really. you know what cryptography is right? it means in order to make use of the original text you need to unlock it by giving it a key which is another unique chain of text. the computer then takes the key and performs MATH along with the gibberish text to bring it back to a readable original. this is not much different than a cereal box decoder ring

well the thing that's great about these situations is if anyone changes something in the gibberish text, the resulting output when you decipher it will be gibberish too. that means people can't tamper with the coded stuff, unless they go through a lot of trouble to pretty much try every possible key to find the one that works with the MATH and produces a valid readable output