What's the rule against perpetuity?

According to Wikipedia, the rule against perpetuity seeks to prevent people from using legal instrument like will, deed, to exert control over the ownership of private property for a time long beyond the lives of people living at the time the instrument was written. The rule seeks to ensure that a testator does not dictate what happens to properties for a period of more than twenty years and to ensure that the present generation can always dictate how to use property. It basically prevents the hand of a dead man from dictating what the use of property is going to be like for the living long after his death.