First, this article from Trustwave does an excellent job covering the topic thoroughly from a beginner's perspective (note: there are formatting errors throughout the article where it mashes two words together every two lines or so, but that aside, the content is good):
https://www.trustwave.com/Resources/SpiderLabs-Blog/Basic-Packers--Easy-As-Pie/
Next, combined with the aforementioned article, the Wikipedia page on the topic will help you flesh out the correct terminology to use for your future searches--things like "executable packing," "executable compression," etc.:
https://en.wikipedia.org/wiki/Executable_compression
Those two resources should give you all you need to start building a solid understanding of the topic.