What does Montag’s description of the TV audience mean? (“He imagined thousands of faces with gray colorless eyes, gray tongues, and gray thoughts looking out through the numb flesh of the face.”)

Montag's unflattering description of the TV audience is a stark illustration of the condition to which people in this dystopian society have been reduced. This is a society in which the government actively discourages people from thinking for themselves. Citizens are deliberately kept in a state of permanent ignorance and passivity by the government through mind-numbing commercials and trash TV. This way people are much less likely to rise up and challenge the regime as they simply don't know any better.
The burning of books and the serving up of an endless diet of bland, mindless entertainment has created a population that is almost completely devoid of independent, creative thought. That's what Montag means when he refers to "gray thoughts." And as the people have been deprived of the best means of developing critical thinking skills—reading books—they blindly accept the regime's distorted vision of reality without a moment's thought.
All individuality has been snuffed out by the government. Virtually the whole population has been moulded into one gigantic gray mass, each person practically indistinguishable from the other. In this nightmarish world, where everything has been throughly homogenized, almost everyone has been brought down to the same gutter level of ignorance, and TV has played the leading role in encouraging and facilitating that ignorance.