A more thorough explanation:

Definition: A digest is an index of legal propositions that shows which cases support each proposition. It is a collection of summaries of reported cases, arranged by subject and subdivided by jurisdiction and court. The purpose of a digest is to make the contents of reports available and to separate, from the great mass of caselaw, those cases bearing on some specific point.

For example, the American Digest System covers the decisions of all American courts of last resort, state and federal, from 1658 to present. It is an important tool for lawyers and legal researchers to find relevant cases and legal propositions.

In civil law, a digest is a compilation and systematic statement of the various areas of law. The most famous example is the Pandects of Justinian, which is a complete legal code of the Roman Empire, together with commentary. The Pandects consist of 50 books and were first published in A.D. 533. They distilled the substance of 2,000 treatises into this abridgment, reducing 3 million lines to 150,000. This prodigious amount of work was carried out in three short years by a commission of 17 jurists headed by Tribonian.

Overall, a digest is a book of secondary authority that summarizes the case law coming within its scope, and its units are summaries of particular points of particular cases. It is a useful tool for legal research and analysis.