A Zen Garden represents the human mind.
Large stones sit in the garden as if rising up out of the pebble carpet. Around these large stones are usually ripple patterns in the pebble carpet, representing movement or a disturbance in the pebble carpet caused by their uprising from beneath the carpet layer.
Everywhere else that the large stones do not ‘rise up’, the pebbles are usually set out in meticulous lines, representing order or harmony.
You contemplate a Zen Garden from a balcony. Sometimes this balcony is entered through a special gate house, where you take off your shoes before entering.
The idea is that each large stone represents a thought arising in the mind. But what you must realize (sic.), is that you are not the large thought arising. Neither are you the ripples and disturbances caused by the arising of the large thought. Neither are you the bliss represented by the ordered pebble carpet, because if you identified yourself with that bliss (or nirvana), it would just be another large thought, and not the same thing as that bliss itself. You are instead the person who observes all of these things, gets up, turns around, and walks out.
Over time it is possible to still the mind in sitting meditation enough that when you do have a large thought arise, no other thoughts surround it. In this state of mental clarity it is possible (though difficult) to investigate that thought a little bit.
This is not the object of meditation, and not really something to distract oneself with. But occasionally it just happens.
Sometimes when it happens, reflecting on the first large thought encourages the arising of lots of other thoughts, and before too long there is no clarity and calm; only monkey mind.
Provided the still and clear state is maintained, however, it is possible to just look deeply into the one large thought which has arisen.
Again, it is easy to start judging it as a good, bad, wise, or silly thought. This, again, is not much use as an exercise. This is not what I mean by investigating a thought.
But it is possible to ask yourself, what is behind this thought? If I acted on this thought, who would gain what? What would I stand to gain?
Through such questioning, it is possible to notice that most thoughts – perhaps as much as 90% of thoughts* – are not relevant at all to things which must be done today. Instead they are supporting some kind of drama, or story, or power game that you (we all) are involved in creating.
And so it is that the mind (the ego) is almost always pursuing some kind of power game; some kind of pursuit towards grandeur (even if it is negative grandeur).
In the same way, some say “what is ego anyway?” or “how is it possible to function as a human being without ego?’, and that is because it is very difficult to overcome the ego. In terms of the pursuits of the conventional mind, the ego is taking up so much mental energy that it becomes impossible to imagine one’s self without it.
*I believe Eckhart Tolle suggests something like 98%.