The unicorn analogy is used to discuss what makes something a reality. If one person sees something and no one else witnesses the same thing, then the person may question the reality of what he saw. If two people see the same thing and agree on what they saw, the reality is more firm. If five people all see the same thing it is even more real. Things aren't real until the majority of people agree on the essence of a thing. As Rosencrantz says, "the more witnesses there are the thinner it gets and the more reasonable it becomes until it is as thin as reality, the name we give to the common experience."
Think about it from your personal experience. When you see something unusual you immediately ask everyone around you, "Did you see that?" You are looking for the confirmation of reality. You do not go around questioning every THING for its reality. We all understand a desk is a desk, a pencil is a pencil etc. That is the collective creation of reality.
The unicorn analogy is a perfect example of how when rare encounters, if experienced by too many or if experienced too many times, get reduced to nothing significant.
"A third witness, you understand, adds no further dimension but only spreads it thinner"
Example, take your favourite song. Listen to it fifteen times in a row. Relax for the day and listen to it again tomorrow. Ask yourself, is the song still beautiful? Most likely you will ask yourself why you even liked it so much in the first place.
Too much of today's pop culture can be related to the unicorn analogy, or the horse-with-an-arrow-in-its forehead analogy
Hope I helped :)