Meditation is a spontaneous state of being that allows us to see who we really are. We begin to witness our mind, observing the habits and thought processes (samskaras) that impact our actions. When we become a witness and see our own consciousness as it unfolds, we are no longer slaves to the mind.
(Related Question: How can I tell if I am "in" meditation?)
To reach a state of dhyana (meditation), which is yoga’s seventh limb, we must train the mind through perceiving our own consciousness. This may be achieved through yoga's fifth and sixth limbs: pratyahara and dharana.
Pratyahara, or withdrawal of external consciousness, is the fifth limb of yoga. It is said to be yoga’s inner dimension, allowing us to turn our attention inward and no longer unconsciously react to external stimulus. To release mental tensions, we withdraw from our external consciousness and thereby achieve true relaxation. Just imagine consciously choosing to not scratch an itch, or to not turn our head when we hear a noise that sparks our curiosity. We can pause and instead observe our impulses, reactions, and desires, separating our inner dimension from the external world.
The main objective of pratyahara is to prepare for the sixth limb of yoga — dharana, or concentration. This is the practice of keeping the mind fixed on a single focus. This can be achieved by chanting mantras, using mala beads, or any other tool to foster about a single internal focus.
We often look for peace and tranquility externally when in truth we can only cultivate it from within. Understanding this through our own experiences with meditation provides tremendous mental benefits and creates the awareness of the power we have over our own consciousnesses. We can find ourselves understanding and detaching from our feelings of anger, sadness, love and other attachments by simply bearing witness to our process.
(Related Question: What can meditation tell me about myself?)