Encryption is used to keep data secret. In its simplest form, a file or data transmission is garbled so that only authorized people with a secret "key" can unlock the original text. If you're using digital devices then you'll be using systems based on encryption all the time: when you use online banking, when you access data through WiFi, when you pay for something with a credit card (either by swiping, inserting or tapping), in fact, nearly every activity will involve layers of encryption. Without encryption, your information would be wide open to the world – anyone could pull up outside a house and read all the data going over your WiFi, and stolen laptops, hard disks, and SIM cards would yield all sorts of information about you – so encryption is critical to make computer systems usable.
An encryption system often consists of two computer programs: one to encrypt some data (referred to as plaintext) into a form that looks like nonsense (the ciphertext), and a second program that can decrypt the ciphertext back into the plaintext form. The encryption and decryption are carried out using some very clever math on the text with a chosen key. You will learn more about these concepts shortly.
Of course, we wouldn't need encryption if we lived in a world where everyone was honest and could be trusted, and it was okay for anyone to have access to all your personal information such as health records, online discussions, bank accounts and so on, and if you knew that no one would interfere with things like aircraft control systems and computer controlled weapons. However, information is worth money, people value their privacy, and safety is important, so encryption has become fundamental to the design of computer systems. Even breaking the security on a traffic light system could be used to personal advantage.