This verse is part of the story of Ehud, a left-handed judge (leader) who delivers Israel from Moabite oppression. Let's break down the meaning:

"But he himself turned back from the stone idols that were by Gilgal..." Ehud has traveled with a tribute to Eglon, the Moabite king, who ruled over Israel. He has intentionally gone past the location of idols. He then claims he must have forgotten a small gift and turns back. This detail is important because it establishes that the following message will be delivered when they are in private.

"...and said, “I have a secret message for you, O king.” This is Ehud's deceptive statement designed to get him alone with Eglon. The phrase "secret message" suggests something important and confidential, naturally piquing the king's interest.

"The king said, “Keep silence!”..." Eglon recognizes the importance of the message and orders those nearby to be quiet, indicating it's a matter he wants to hear in private and that Ehud might want to deliver it privately as well.

"...All who stood by him left him." This is the key outcome of Ehud's ruse. Because the king has commanded silence, all the attendants and guards leave the chamber, leaving Ehud alone with the vulnerable King Eglon. This sets the stage for Ehud to assassinate Eglon and spark the rebellion that will free Israel.

In essence, this verse describes a clever tactic used by Ehud to isolate King Eglon, gaining the opportunity to carry out his plan. It highlights:

Deception: Ehud is using trickery to get close to the king.
Strategic Thinking: He understands the power dynamics and how to manipulate them.
Preparation: He has likely planned every step to create this opportunity.

The verse is pivotal in the narrative because it paves the way for the central event: the assassination of the Moabite king.