It's difficult to decypher legal abbreviations sometimes, and especially when doing research on historical, foreign, comparative, and international law topics. For unknown legal abbreviations, you can use Black's Law Dictionary (online on Westlaw) and Prince's Bieber Dictionary of Legal Abbreviations. But here's a handy free resource that helps with many legal abbreviations you'll encounter - the Cardiff Index to Legal Abbreviations.
For example, if you found a citation to Shelley's Case - 1 Co. Rep. 93b, 76 Eng. Rep. 206 (1581) - you'll see in the Cardiff Index that the abbreviations stand for "Coke's King's Bench Reports" and "English Reports". The reports of Sir Edward Coke are in Eighteenth Century Collections Online (ECCO) and Early English Books Online (EEBO). The English Reports are in HeinOnline, LLMC Digital Law Library, and CommonLII (free).