This guide includes a combination of primary, secondary, and tertiary information. While what is considered primary and secondary differs by discipline, in general:
Primary: Original materials that have not been filtered through interpretation or evaluation by a second party. These range from historical documents from the time of a past event, letters, memoirs, recipe cards or cookbooks (in some cases), or pieces of original scholarly research.
Secondary: Sources that contain commentary on or a discussion about a primary source. This is what a lot of books and articles are considered, especially if they are discussion someone else's ideas or summarizing research in a certain area.
Tertiary: An index and/or textual consolidation of primary and secondary sources (e.g. encyclopedias). In this guide, these are largely going to be found under "Reference" sections. You can also think of these as background.
Tertiary information is the best place to start, as it gives you a broad overview of a topic. Once you have a grasp of a topic, you will be able to more effectively search for primary and secondary material related to your topic.