Decryption is taking encoded or encrypted text or other data and converting it back into text you or the computer can read and understand. This term could describe a method of unencrypting the data manually or unencrypting the data using the proper codes or keys.
Data may be encrypted to make it difficult for someone to steal the information. Some companies also encrypt data to protect company data and trade secrets. If this data needs to be viewable, it may require decryption. If a decryption passcode or key is unavailable, special software may be needed to decrypt the data using algorithms to crack the decryption and make the data readable.
While it's possible to crack all forms of encryption, it requires a tremendous amount of computing power in addition to time. For example, to break something encrypted with AES-256 bit encryption would take millions of years to break with today's computer technology. However, future technologies (e.g., quantum computer) could break encryption a lot faster.
The act of decryption may also be called unencrypt or unencryption.