What makes a demonstrative adjective different from a regular adjective is that it indicates precisely which person, place or thing is being referred to. For example, instead of describing the table's glassiness, we can refer to it with reference to the person being spoken to. We can say, "this table", which is near us both, "that table", which is only near you, but not me, or "that table over there", which is far from both of us.