Background sources (also known as reference sources) are tertiary and contain information we "refer" to, such as dictionaries and encyclopedias. These are good sources to use to get started - when you need ideas for how to narrow your topic, or could use more words to describe your needs (such as when thinking of keywords to put into a search box.)
Credo Reference offers background information on topics from hundreds of full-text general and subject-specific reference/encyclopedia titles, as well as 500,000+ images and audio files and over 1,000 videos.
Trusted overview information, including Merriam-Webster’s Collegiate Dictionary, magazines and periodicals, and other research, written by Nobel laureates, historians, curators, professors, and other notable experts.
This encyclopedic entry on Labor Movements from The Oxford Encyclopedia of American Business, Labor, and Economic History, could be helpful as an overview of the important dates, names and events in labor history. These kinds of entries can help you get ideas for how to narrow the "focus" of a topic that may be too broad, or give you additional "keywords" to use in your online searching.
From the Opposing Viewpoints database, read articles from magazines, peer reviewed journals, and encyclopedias; read opinion pieces about the pros and cons of unions; listen to news stories from NPR; watch videos from NYTimes.com - all about labor.