There is a famous quote from Karl Marx that says that religion is the opiate of the masses.  However, in this book, the phrase is "opiate of the massives."  Freak gets it a little bit wrong.

Instead of religion, Freak is talking about TV.  He says that it is the opiate of the masses.  What this means is that it is a drug that lulls them into being dull and not really caring about things.  (That's because an opiate is a drug like opium or morphine that serves to dull a person's senses.)  He says that "massives" are fat heads and that TV is their opiate.