Secondary sources include things like legal dictionaries, legal encyclopedias, treatises, and law review articles. Essentially they are materials written about the law, rather than the law itself, and can be a great place to get started on your research. They can also help you to research more efficiently and save a great deal of time.
Secondary sources can be used to find:
Secondary sources are not law, and as such are not mandatory authority. They are always persuasive authority.
Persuasive authority can be used to support a legal position or argument, and courts may choose to consider it when deciding an issue. However, it is not mandatory that they do so.
Deciding whether to cite a secondary source can be tricky. Some treatises are very highly regarded and can be cited in your briefs and memos, but other sources, such as encyclopedias and ALRs, generally should not be cited at all. If you are ever unsure about a source you should check with your librarian or instructor.
Black's Law Dictionary by Legal dictionaries provide definitions for unfamiliar legal terms, and may include references to cited authority and West Key Numbers.
Ballentine's Legal Dictionary/Thesaurus by 
Prince's Bieber Dictionary of Legal Abbreviations by Legal abbreviation aids can help youhelp you to interpret citations and make sense of all of abbreviations used in the legal world.