This precept points to the various modes of meditation and also other practices used to strengthen mental discipline. A very common practice is "Noticing One's Breath", in which, sitting comfortably with your back upright, you notice your breath as it goes in and out, in and out. You also come to notice that your mind is a nonstop whirlwind of disjoint thought, and with continued meditation the mind tends to calm down and clear up.
It's been said that the main thing that the Buddha introduced to the spiritual practices of his day was the practice of Vipassana meditation (or noticing-the-breath-meditation) in order to bring about enlightenment.
Congratulations! You are now enlightened! Just kidding. Now go meditate.