For someone that is truly a beginner in cryptography it is helpful to understand at a very basic level how encryption/decryption works.
Using something simple as a Caesar cipher or ROT13 you can show how plaintext undergoes a transformation using an algorithm to create the cipher text and how that same algorithm is used to convert the cipher back to plaintext.
This simple idea provides the basis to discuss other cipher techniques (and their shortcomings); as well as provide an early example of symmetric-key algorithms.
There is zero cryptography value in spending more than 5 minutes on a Caesar cipher or ROT13. It's a shame that so many university courses on cryptography even waste such time on such topics. It would be like teaching times-tables in a course on calculus.
Using something simple as a Caesar cipher or ROT13 you can show how plaintext undergoes a transformation using an algorithm to create the cipher text and how that same algorithm is used to convert the cipher back to plaintext.
This simple idea provides the basis to discuss other cipher techniques (and their shortcomings); as well as provide an early example of symmetric-key algorithms.