If you've ever tried to log into a web app unsuccessfully, you may have seen a error page that contained the word "Shibboleth". In this case, it's referring to the single-sign on authentication app, but in more general terms, a shibboleth is an indication that you're a member of a group. This might be a certain word, pronunciation, or a concept with which you're familiar, but are opaque to outsiders
Gilead then cut Ephraim off from the fords of the Jordan, and whenever Ephraimite fugitives said, 'Let me cross,' the men of Gilead would ask, 'Are you an Ephraimite?' If he said, 'No,' they then said, 'Very well, say "Shibboleth" (שבלת).' If anyone said, "Sibboleth" (סבלת), because he could not pronounce it, then they would seize him and kill him by the fords of the Jordan. Forty-two thousand Ephraimites fell on this occasion.