2.19 identify common materials which are electrical conductors or insulators, including metals and plastics

Electrical conductors are materials that allow a current to pass through them. To do this they need to have 'free' electrons, because current is a flow of electrons. Metals have free electrons because of the way they are bonded (atoms and electrons within a lattice) this means they are good electrical conductors.
Plastics are polymers which are bonded in a way that means electrons aren't free and so can't move. No flow of electrons means no electric current so they are insulators.