"it is a sin to think words no others think and to put them down upon a paper no others are to see... There is no transgression blacker than to do or think alone" What does Rand mean by this? What could motivate such an idea?

Rand's whole society works on the idea of mass conformity. When there are is no individuality, there will be no competition or inequality and hence peace. New ideas lead to human expression and ingenuity. The leaders don't like this because it threatens their power and hold over the population.