What does Judges 12:6 mean?

they’d tell him, Say ‘Shibboleth.’ If he said ‘Sibboleth,’ because he couldn’t pronounce it right, they’d grab him and kill him at the river. Forty-two thousand Ephraimites died that day. Judges 12:6 - Modern Text Bible

(They used a word as a test, since Ephraimites pronounced it differently. This led to a huge loss of life.)