5.15 understand that an addition polymer is formed by joining up many small molecules called monomers

A polymer is just a very long saturated chain (saturated as it has no carbon double bonds). Small molecules called monomers join together to make polymers. One type of polymer is an addition polymer. The monomers that make up addition polymers are alkenes as they have a carbon-carbon double bond. If alkenes are put under a high pressure with a catalyst, they will break their carbon-carbon double  bond and polymerise (join together), forming a polymer.