Decryption is covered in full, together with its counterpart, in the entry Data Encryption and Decryption, which explains the main methods, key management and their use in banking and payments. This page gives a short definition and explains why the two terms share one entry.
Decryption is the process of turning scrambled data, known as ciphertext, back into readable plaintext. It uses the same cipher that protected the data and a key that matches the one used to lock it. In symmetric systems the same secret key both encrypts and decrypts; in asymmetric systems, data locked with a public key can be decrypted only with the matching private key. Banks, card networks and payment apps decrypt data constantly, for example when a payment message arrives at its destination or when stored card data is retrieved inside a hardware security module. Without the right key, well-designed encryption cannot be reversed in any practical time, which is why stolen encrypted data is far less useful to criminals than stolen plain data.
Encryption and decryption are two halves of one process, so most references, including this site, treat them together. The broader science is described under cryptography, and the locking step on its own under encryption.