Encryption is a cryptographic primitive: it transforms a plaintext message into a ciphertext using a cryptographic algorithm called a cipher. Encryption in modern ciphers is performed using a specific algorithm and a secret, called the key. Since the algorithm is often public, the key must stay secret if the encryption stays secure.
Without knowing the secret, the reverse operation, decryption, is mathematically hard to perform. How hard depends on the security of the cryptographic algorithm chosen and evolves with the progress of cryptanalysis.