Meditation is the practice of turning your attention to a single point of reference. It can involve focusing on the breath, on bodily sensations, or on a word or phrase known as a mantra.
In other words, meditation means turning your attention away from distracting thoughts and focusing on the present moment. Meditating is deceptively simple, and very sophisticated at the same time.
A cartoon from The New Yorker sums it up: Two monks are sitting side by side, meditating. The younger one is giving the older one a quizzical look, to which the older one responds, "Nothing happens next. This is it."