The property of a material that allows it to be a good conductor of electricity is its ability to allow the easy movement of electric charges, specifically electrons. This property is known as electrical conductivity.
In materials with high electrical conductivity, electrons are able to move freely through the material's atomic or molecular structure. This movement of electrons constitutes an electric current. Metals are typically excellent conductors of electricity due to their unique atomic structure, which allows for the relatively unrestricted movement of electrons.
In metals, the outermost electrons of atoms (valence electrons) are not tightly bound to individual atoms and are instead shared across a lattice of metal atoms. This "sea" of delocalized electrons can move freely in response to an applied electric field, allowing for efficient transmission of electrical energy.
Materials with lower electrical conductivity, like insulators, have electrons that are tightly bound to their atoms and do not move easily in response to an electric field. Semiconductors have intermediate conductivity and can conduct electricity under certain conditions, such as when subjected to higher temperatures or with the addition of impurities (doping).
In summary, a good conductor of electricity has the property of high electrical conductivity, which allows electrons to move easily through the material, facilitating the flow of electric current.