You have already studied that the mind is essential in the process of acquiring knowledge. For example when you are listening to a speaker in lecture, you learn through your ears. But if your mind is not concentrating on the lecture then you won't understand anything there. As the mind is unstable, it will run away from the lecture. So to learn what the speaker is telling, you must establish control over your mind and force it to listen to the lecture. Then only you'll be able to acquire knowledge.

Hence you have to control movements of mind and make it stable whenever you want, as per your requirements. But this is the most difficult part because of unstable nature of mind. This process of making mind stable is called concentration.
This is true for any function of mind. If you want to remember a particular event, you will have to concentrate your mind till you remember that event.

It is clear from the above discussion that it is essential to control the mind. Only then, one can get what he wants. But controlling mind is the most difficult task.
Efforts are being continuously made to find out the ways and means to establish control over mind. Meditation is one of the very effective ways to control the mind.
This control can't be achieved in short period. Meditation is a systematically designed technique to achieve this step by step.