A polymer is a very long molecule made from many small units called monomers joined together in a chain. Polymers are not only man-made plastics. Living things make natural polymers too, and three of them matter here.
DNA is a polymer made from four different monomers called nucleotides. The order of the four types in the chain carries the genetic information of a cell.
Starch is a polymer based on sugars. Plants make it by joining many sugar units together to store energy. Proteins are polymers based on amino acids. Many amino acids join in a chain, and the order in which they are joined decides which protein is made.