Shepard's allows you to track the citation history of a court case. When you "Shepardize" a case, you will see all of the other cases that have cited that case, and if they treated the case favorably or unfavorably. This is how you can tell if the law in your case is still considered "good law" or if it has been overturned or challenged by other cases.
Shepard's is only available through Nexis Uni, and has been around for more than 100 years. It is a very common and important process in legal research.
To start, make sure you have the citation for your case, then find the case in Nexis Uni.
Once you have searched for and located your case, you will see a small symbol at the top of the case page. This symbol indicates how you should proceed with using the case law -- positively, with caution, or stop and evaluate.
To Shepardize the case, on the right side of the screen, you can view the top citing reference and all citing decisions. You can also use the option to "Shepardize this document".
To see a visual demonstration of this process, view the video below.