Meditation can mean either clearing your mind of all thought, or focusing on one thought, both done in an effort to achieve enlightenment. For example, in some masters of "zen" or "chan" buddhism will give thier students koans to meditate on, such as, "What is the sound of one hand clapping?". On the other hand, many other sects teach thinking of nothing. Either way can be effective for those who engage in the activity not for religious enlightment, but simply to relax and gain a greater understanding of life.
He medidated on the koan and understood that the lack of an answer was in itself an answer, and yet not the fullness of the answer.
I engage in meditation for a few minutes before I go to bed to help settle my mind.
It is not to clear your mind off all things but one. It is to clear your mind of all things. To focus on the lack of thoughts that come between thoughts, and to lengthen this period to several minutes, even hours. It is only then that you can truly listen to your inner self. If you are focusing on a tree or a flower, you cannot focus on your inner self.
Meditation is something that is very misunderstood by westerners.