Magento 2.0.18 is the final 2.0.x release. After March 2018, Magento 2.0.x will no longer receive security patches, quality fixes, or documentation updates. To maintain your site's performance, security, and PCIPayment Card Industry: Refers to debit and credit cards and their associated businesses. compliance, upgrade to the latest version of Magento.
The encryption key is used to protect passwords and other sensitive data. During the Magento setup, you have the option of generating an encryption key, or using an another.