This verse is spoken by Jonadab to King David after David learns of Amnon's death. Amnon, David's eldest son, was murdered by Absalom, another of David's sons, in revenge for Amnon raping Absalom's sister, Tamar.

Here's a breakdown of what the verse means:

"Now therefore don’t let my lord the king take the thing to his heart...": Jonadab is urging David not to be overwhelmed with grief and despair. He's telling David not to let his emotions control him.

"...to think that all the king’s sons are dead...": This is the key point. Jonadab is implying that there's a rumor or a fear that Absalom, in his rage, has killed all of David's sons. David, overcome with grief over Amnon, may be fearing the worst.

"...for only Amnon is dead.": Jonadab is reassuring David that Amnon is the only one who has been killed. He's trying to dispel the rumor and alleviate David's fears that the situation is even worse than it is.

In essence, Jonadab is trying to console David by clarifying the extent of the tragedy. He's saying, "Yes, Amnon is dead, which is terrible, but don't let your grief lead you to believe that all your sons are dead. The situation is not as catastrophic as you might think."

This verse is important because it shows:

The extent of the turmoil in David's family: The rape, the murder, and the subsequent rumors highlight the chaos and violence brewing within David's household.
Jonadab's cunning: He quickly assesses the situation and tries to manipulate David's emotions. Jonadab, who is known for being shrewd, is trying to prevent David from taking drastic actions based on inaccurate information.
The devastating impact of Amnon's actions: His sin has triggered a chain of events leading to tragedy and grief for David and his entire family.