The document discusses the process of migration, where substances diffuse from packaging materials into food. Migration occurs as chemicals are not chemically bonded to polymer molecules and can freely move within the polymer matrix. The rate and extent of migration depends on factors like the concentration and properties of migrants in both the packaging and food, the type of polymer used, storage time and temperature, and food properties such as fat content. Specific examples given are bisphenol A migrating from plastic bottles and cans into food, and diethylhexyl adipate migrating from PVC cling films into fatty foods.