Opcode Cache for Dummies

PHP is an interpreted language. This means that each time a PHP generated page is requested, the server must read in the various files needed and "compile" them into something the machine can understand (opcode). A typical Drupal page requires more than a dozen of these bits of code be compiled. Opcode cache mechanisms preserve this generated code in cache so that it need only be generated a single time to server hundreds or millions of subsequent requests. Enabling opcode cache will reduce the time it takes to generate a page by up to 90%.

Slaughter, Harry. DevBee (2006). Articles>Web Design>Server Side Includes>PHP