He alone is my rock and my salvation, my stronghold; I will not be shaken.
David in two words has made a huge statement of faith. He's not relying on his riches or his power or his armies. He's relying on God alone as a stronghold. (He certainly wouldn't be relying on the stock markets today.) If anyone had reason up to then to rely on things he had done or accumulated, David was the guy.
"He alone..." But David doesn't do that. And with good reason. If David couldn't see God as his rock, salvation, and stronghold, then I'm not sure who could. His route to the kingship was, shall we say, a little difficult, having spent time running from Saul. David had to wait for his moment to become king. He had to rest in the promise God made to him.
So what does it mean to have God as our rock and stronghold? It means we have stability. It means our current mood/status does not depend on what the economy is doing, what leaders are in charge, or _______. Temporal, material things don't shake us, because we have to place our trust and faith in the eternal Creator. And when we do let things get to us and face stress, anxiety, conflict, etc, we can climb back to our rock quickly.
In verse 8, David gives us advice on making God or rock and stronghold. "Trust in Him at times... pour out your hearts before Him."