A shibboleth is like a motto or catchphrase that members of a group tend to say, like the conservative shibboleth that the only good government is a small government.
Shibboleth is a Hebrew word that means "ear of corn" or "flood." In a Biblical story, the word was used as a password — a means to figure out who was part of your group and who wasn't. It still has that sense of identifying someone as a member of a group. Sometimes it also means "platitude" or "truism," a phrase that is so common everyone thinks it's true, like "crime doesn't pay," or "better late than never."