The corn in the phrase corned beef refers to salt-curing, or brining beef to preserve it. The corn refers to the large grains of salt used in curing the beef. The meat is placed in a crock and liberally covered with large grains (or corns) of salt.
Etymologically, the English word corn comes from the Germanic root kurnam, used to refer to small seeds or kernals, cognate with kernal and with Latin grain. Corned beef, traditionally made with a brisket of beef in Eastern European and Jewish traditions, is fairly simple to do at home. Here’s a recipe from Alton Brown for making your own corned beef.