Most plastics contain organic polymers.The vast majority of these polymers are formed from chains of carbon atoms, 'pure' or with the addition of: oxygen, nitrogen, or sulfur. The chains comprise many repeat units, formed from monomers. Each polymer chain will have several thousand repeating units.
The backbone is the part of the chain that is on the "main path", linking together a large number of repeat units.
To customize the properties of a plastic, different molecular groups "hang" from this backbone. These pendant units are usually "hung" on the monomers, before the monomers themselves are linked together to form the polymer chain. It is the structure of these side chains that influences the properties of the polymer.
The molecular structure of the repeating unit can be fine tuned to influence specific properties in the polymer.