Reference works (encyclopedias, dictionaries, handbooks, manuals, directories, bibliographies, concordances, etc) introduce key concepts.
They typically provide definitions and / or overviews as well as citations for important sources (both primary and secondary) on a particular topic.
They also assist with navigating other sources for your research, such as sacred texts.
Reference works are an excellent place to begin your research on an unfamiliar topic or to answer topical questions that have come up in the course of your research.