Man is like grass and soon withers.
To understand this passage in Daniel, it is helpful to understand its biblical context and its historical context. In chapter 3, King Nebuchadnezzar rebels and sets up an idol in God's place. In chapter 4, God humbles Nebuchadnezzar, and he repents. In chapter 5, the tyrant Belshazzar refuses to humble himself before God and dies that very night. Darius then takes over the kingdom and establishes Daniel in authority.