Call back help!

I just can not seem to wrap my head around proper implementation of callbacks. In this situation I am finding physical drives attached to the system. Displaying them on a webpage. Users select which drives they want to update. That array of objects is sent to the server, via AJAX. from that array of objecst I get the MODEL value of each drive and determine if there is firmware for the drives. In order to determine which firmware goes to each drive I need to read portions of each file in a directory and determine it applies to a model number. Once I find a match I need to determine if that firmware file's firmware value is an actual firmware upgrade. If it is, I will store somehow, tbd, if it is not, I will notify the user there is NOT a firmware file for that drive. Once that has been unwound, executing firmware update providing a means to update all the drives to their max available firmware. It seems reading the files I need a callback and the process that first must determine which files apply to a particular model has a callback that eventually returns to the usre. So...Help me understand callback? I have looked and looked trying to get this.