When data is stored at rest on a hard drive, there are ways to encrypt it

In windows OS the tool is BitLocker and EFS (encrypting file system)

On Mac OS the tool is FileVault

Data in a database may also be encrypted by column or record

Another place encryption is important is when transferring data across a network

In the browser, the tech is HTTPS, SSL, TLS

For encryption and decryption to work, both sides must agree on a standard/same algorithm.

Popular standards are DES (data encryption standard) and AES (advanced encryption standard) as well as the size, e.g. 128 or 256 bits.