Namaste is both a spoken word and a performed gesture. It speaks of our inner valuing of the sacredness of all. It expresses our intuition that all souls are divine in their essence.
It is saying silently, "I see the Deity in us both and bow before him/her. I acknowledge the sacredness of even this mundane meeting and I cannot separate that which is spiritual in us from that which is human and ordinary".
For a more complete (and fasinating) description of the greeting lookHere