Encryption is transforming information through a cipher to protect it from unauthorized access.
Encryption is the process of taking some information (the plain text), and transforming it (usually with a cypher and a key) so that it is secure. Assuming a sufficiently strong cypher and key have been chosen, this cyphertext can be shared widely, without the information being compromised.
The reverse of Encryption is Decryption, where the process is reversed (normally by someone holding the key and cypher details) to return the information.
See the Wikipedia Page for more details.