Encryption is the process by which data is made illegible and unusable for unauthorized viewers. In order to use or read encrypted data, you must decrypted will. A secret key is required for this. There are two types of top-level encryption: symmetrical and asymmetrical .
In the symmetric encryption the same key is used to encrypt and decrypt the data. Consider using a password manager desktop application. When you enter your passwords, they are encrypted with your personal key (this key is often derived from your master password). When the data needs to be retrieved, the same key is used and the data is decrypted.
In the asymmetric encryption a key pair with a public and a private key is used. Both keys can be used for encryption, but only one key cannot decrypt your own encrypted data. To decrypt you need the other key of the pair. Asymmetric encryption is used, for example, for Transport Layer Security (TLS) (for HTTPS) and data signatures.