An encryption key is a piece of information that can encode or decode cryptographic information. It is usually a string of numbers or letters that are stored in a file.
An easier way to understand this would be that it is a string of code that is used to scramble or unscramble data.
They are designed so that only certain people can read or access that data.
Encryption uses complex algorithms to scramble the data that is being sent. Once received, the data can be decrypted using a key provided by the originator of the message. This allows Monero users to safely send Monero from one wallet to another.
Every encryption key is unpredictable and unique, making it almost impossible to hack or copy.
Private Keys are encryption keys that remain in the confidential use of the original two users. The risk here is that the key can be lost making the system unusable.
Public Keys are encryption keys that are publicly available via an accessible directory.
There is less risk of loss because this key is publicly held.