What does Judges 3:19 mean?

Ehud (Judges 3:15–18) is on a mission to kill the king of Moab. As the one selected to bring Israel's tribute to the king, Ehud has gained an audience with Eglon at his palace in Jericho. That has already taken place, and Ehud has begun the journey back home with those who carried the tribute. This is not a missed opportunity, but the exact situation for which Ehud has planned.

He stops, sending the others on their way. Ehud turns back toward Eglon's palace at a place where some idols stood near Gilgal. These idols may have been part of the pagan worship that had taken root in Israel (Judges 3:6). In any case, they had become a landmark in the region. Gilgal was only about two miles from Jericho. Archaeologists in the 1930s uncovered the remains of a large structure in Jericho now believed to be the palace of Eglon. The dimensions were about 40 by 50 feet, or 12 by 15 meters, and the site included expensive pottery and a cuneiform tablet.

Ehud gains another audience with the king by reporting that he has a secret message for Eglon. This, apparently, is how he explains returning to the palace after leaving with his companions earlier in the day. Eglon wants to know the message. He commands "silence;" in this context, that means leaving the room empty so no one else can hear the secret. Attendants and guards leave the room and, apparently, close the door behind them. This leaves Ehud alone with Eglon the king.