Cryptology is the science of making and breaking secret codes. These are used in computers to keep information secure and where people that should not know your information do not learn it.
There are several areas in cryptology. Two of the major areas are cryptography and cryptanalysis
Cryptography is the area of creating and using codes. We use the various methods learned here to encrypt our messages sent across the network and to store our data on our computers. By encrypting them, they can only be read by people that have the correct keys. This has been done way back in history to protect messages. Julius Caesar encrypted his messages to his generals so his enemies would not know his plans. We now do it much more complicated than he did but the goal is the same. You want your messages with your credit card number when you buy stuff encrypted. You know your computer is encrypting the messages when you see the closed padlock ion your browser.
Cryptanalysis is learning how to break codes and read messages. You may think this is just a criminal side but it is also important on the side of encrypting. People can test the various methods of encrypting and making messages secure by seeing how soon they can break the code. The longer it takes to break the code the more secure the code is. No encryption code is 100% safe. The question is how easy is it to break the code. Cryptanalysts are always testing that so we know what are safer methods and which are no longer safe. The faster and bigger computers get the less reliable methods are.