<p>I am trying to learn some programming related terms, and I often come over the word &quot;handler&quot;. Can anyone please explain what it means and when to use it?</p>

A handler is a routine/function/method which is specialized in a certain type of data or focused on certain special tasks.
Examples:
Event handler - Receives and digests events and signals from the
surrounding system (e.g. OS or GUI).
Memory handler - Performs certain special tasks on memory.
File input handler - A function receiving file input and performing
special tasks on the data, all depending on context of course.