The Magento Encryption Key is generated during the Magento Installation. It is used for the encryption and the security storage of the sensitive data in the script's database.
The Magento Encryption Key is kept in the app/etc/env.php file. You can open that file by using the File manager tool in your cPanel for example. In this file, you should look for the following code:
'key' => '358908865aba76fcd9e1859578e9285a',