What does Psalm 3:5 mean?

In the middle of a dire threat, David is praying to express his faith that God will ultimately deliver him (Psalm 3:1–4). This specific psalm relates to the threat of David's own son, Absalom, who was counseled to send a 12,000-man army to kill his own father (2 Samuel 17:1–4). That plan was narrowly thwarted by Hushai, an ally of David (2 Samuel 17:15–16). This is not the first time David has encountered danger, however (Psalm 18:3–6). Recalling a prior stressful event when his enemies threatened his life, David says he "lay down and slept." Although David was surrounded by his enemies, he felt secure because the Lord was on his side. Psalm 91:14–15 promises: "Because he holds fast to me in love, I will honor him; I will protect him, because he knows my name. When he calls to me, I will answer him; I will be with him in trouble; I will rescue him and honor him."

Daniel 6 relates the story of how King Darius' high officials and viceroys conspired against Daniel to have him executed. They persuaded the king to pass a law that forbade anyone to ask anything of any god or man except of the king for thirty days. Whoever disobeyed the law would be cast into a lions' den. They knew that Daniel prayed three times a day to the Lord, so they expected him to break the law and be thrown to the lions. Although Daniel knew about the law, he prayed as usual to the Lord and was apprehended and cast into the lions' den. However, God sent an angel to seal the lions' mouths. The next morning the king rushed to the den and learned that Daniel was safe. Daniel testified that God had delivered him. The king released Daniel, but had his accusers and their families cast into the den, where the lions killed them.

We may not always be aware of all the potentially harmful situations from which God delivers us, but we can trust that nothing happens outside of His power or control.