The atomic number of the element copper is 29, which means it has 29 protons in the middle and 29 electrons moving around the outside. (The 29 negative charges of the electrons and the 29 positive charges of the protons balance out, so the atom is neutral when all of its electrons are in place.)
Copper has two electrons in the innermost shell, eight in the next shell, eighteen in the third shell, and one in the fourth shell. This means that the first three shells each have as many electrons as they can hold, and the fourth shell has one lonely electron. (The fourth shell can hold up to 32 electrons.) Because this one lonely electron is all by itself in the outer shell, it can easily separate from the rest of the atom and go roaming around, which makes copper a very good conductor.