Secondary sources are publications which describe, comment on, or analyze the law (as opposed to primary sources, which contain the actual text of the law). Many different types of publications are included within this category: legal dictionaries, legal encyclopedias, American Law Reports, law reviews, Restatements, and treatises. Secondary sources can be valuable at the beginning of a research project, as they can provide a researcher with an overview of an unfamiliar subject area, introduce the key terminology, and provide citations to important primary source materials.
Keep in mind that secondary sources are not the law and cannot be binding precedent (i.e. courts do not have to follow them). Occasionally, certain types of secondary sources can be used as persuasive authority.