This verse, 1 Samuel 20:39, is part of the story of Jonathan and David, two close friends and allies whose loyalty to each other was tested by King Saul's animosity towards David. Let's break down what it means:

"But the boy didn't know anything." - The "boy" here is likely the servant Jonathan used to retrieve the arrows. Jonathan uses a prearranged signal with David. In order not to expose what they were doing, the servant was not in on the scheme. He only knew he was sent to find some arrows.

"Only Jonathan and David knew the matter." - This emphasizes that the signal they were using to communicate danger was secret. The "matter" refers to the situation, the danger David was in because of Saul's anger, and the coded message they were using to indicate whether David should flee or stay. Only Jonathan and David knew the meaning behind the arrow placement and Jonathan's words.

In essence, the verse highlights the secrecy and the trust between Jonathan and David. They had a coded system to communicate, and they were careful not to reveal their plans to anyone else, likely to protect David and perhaps even themselves from Saul's potential wrath.

This secrecy is important because it emphasizes the following:

Danger: The situation was perilous. If Saul had known about their plans, it could have had dire consequences for both David and Jonathan.
Trust: David and Jonathan had complete trust in each other. They relied on each other to keep their agreement secret and to act accordingly.
Loyalty: Jonathan was willing to risk his own safety to warn David of the danger he was in, demonstrating his deep loyalty.

This verse contributes to the overall narrative of their friendship and the difficult circumstances they faced due to Saul's paranoia.