Content handlers, or more specifically content part handlers are similar to event handlers in programming in general: these methods' job is to handle specific events of a content part, e.g. when it's instantiated, deleted or versioned.
You can find a complete list of such events on the "Understanding content handlers" page of the official documentation.
This page is part of the Orchard Dojo Library!
This page and many others here are part of the open-source, portable package of Orchard 1.x goodies: the Orchard Dojo Library for Orchard 1.x. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!
Note that the Library also has an Orchard Core version, check it out here!