Meditation is the Science and Art of doing nothing.
One can enjoy the serenity and joy of meditation only when theirVruttis (traits of the mind) are controlled.
The diffused rays of the sun, when gathered together to a point by using a convex lens, can start a devastating fire. Similarly, when thoughts of the mind, which are normally diffused or confused when are gathered together and concentrated, can give us Knowledge and Power.
Secondly, when the river is flowing wildly, it is very difficult to utilize her water or have a control over her. But construction of the dam makes it serene and one can use the water in a proper way as per required. Same symbol is applicable to our mind. When our thoughts are wild it is difficult to act efficiently but reverse is the case when you have a good control on your mind.