David encourages us to not be agitated or envious of those who seem to get away with wickedness and oppression in the world, because their day is coming. Evil has a shelf life. The righteous may go through hard times, but the Lord makes sure they are never abandoned, forgotten, or overlooked.
We all have pretty solid ideas of what we think is the way work and life should work the best, and when it comes to spiritual things – it's not much different. However, following Jesus looks a lot more like trusting than it looks like "knowing."