The Magento Compilation is a feature that allows you to compile all Magento installation files in order to create a single include path. This feature is designed to improve the performance of your online store and is supposed to give between 25% and 50% increase in your page load speed.
Note that before making any changes to your Magento Installation files, the Magento compilation feature should be disabled. To disable Magento Compilation, go to your Admin Panel an navigate to System > Tools > Compilation and click on the Disable button.