Soygal Rinpoche provided me with one of the best ways of understanding meditation.
He would say that when you sit for meditation that you are like a jar of muddy water. If you let that jar of muddy water calmly sit where it is, the mud will settle to the bottom and clear water emerges. When you sit to meditate, let your thoughts gradually settle. What emerges is clarity of mind.
If you become impatient with that jar of muddy water and try somehow to force the mud to settle at the bottom, clear water never emerges. If you become impatient with your meditation and fight the constant thoughts that flow into and out of your awareness, you will also stay in a state of constant agitation.