IBackgroundTask interface is one of the most simple interfaces you can find in Orchard (aside from the empty marker ones). The only method contained in this interface you need to implement in your class is the Sweep method (without input parameters). Orchard will execute the Sweep method of every IBackgroundTask implementation every 1 minute, so it enables you to simply define some logic that will run periodically. It is useful for recurring tasks and running huge tasks in smaller batches (just like the Orchard.Indexing module does with updating the indices).
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!