To say in few words, its an application just like IonCube which encodes the files and also decodes them to run on the server, if installed.
zend is most powerfull php encoder it will helps to keep your code same from copycats and nullers also lot more uses.
it's a zend product that will make your php codes faster. you can get more information on http://www.zend.com/products/zend_optimizer