FormKey disables "form key" protection introduced in Magento CE 1.8. This module is required when you want to use M-Turbo on Magento CE 1.8 or higher.
M-Turbo is a caching component for Magento e-stores that provides ultra fast response of the site and improves shopper experience.
It boosts Magento speed and overall performance, as well as helps to save significant ammount of server resources. It is an integral part of Magento site optimization. More information at M-Turbo: Magento Accelerator product page.