In these plastics, the chains mainly lie side by side and are not strongly chemically bonded to one another. These plastics are soft and easily stretched out of their original shape. A good example of this is cling wrap, commonly used to cover food.
They generally melt when heated. The heat makes the chains vibrate and so puts extra distance between them, which weakens the bonds between the chains and lets them move past one another. This explains why cling wrap becomes more pliable when it has been in the microwave oven for a period of time.
These plastics are called thermoplastics because they become more pliable when heated (the original meaning of plastic is "capable of being shaped or formed"). They are the most easily recycled plastics.