If you wrap food in plastic wrap so it is in contact with the food surface, the application of heat will release chemicals from the plastic that will leach into the food. These chemicals are known or suspected carcinogens (I can't remember which).
You can buy plastic wrap that is of a different composition and safe in this situation.
If you read the fine print on the box, it should tell you whether or not to use it in heating food.
Of course, if you don't use plastic wrap this way, you don't have to worry.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan