What a conductor is
Charges will not flow through just anything. The materials they flow through easily have a name, and one family of them does it far better than the rest.
Conductors
Materials that allow an electron current to flow through them easily are called conductors.
Every metal is one
All metals conduct electricity well. Copper, aluminium and iron are conductors, and so is every other metal.
A question chapter 3 left open
Chapter 3 told you that metals are good conductors of heat and electricity, and listed it among their properties. It never said why. That is what the next page answers, and the answer turns out to be about where a metal atom keeps its outer electrons.
Conducting is about free charge, not about being solid
A material conducts if it contains charged particles that can move. Chapter 10 made the same point with molten and dissolved ionic compounds — there the moving charges were ions, and in a metal they are electrons.