a valence electron
Of all an atom's electrons, only the ones in the outermost shell get to do chemistry: they are the atom's hands that reach out and grip neighbours. These outer-shell electrons are the valence electrons. The inner electrons are like a packed lunch already eaten and forgotten; the valence electrons are the ones on the table, ready to be traded, shared, or pooled.
The number of valence electrons follows the group (column) in the periodic table: sodium and other group-1 metals have 1, magnesium 2, carbon 4, oxygen 6, chlorine 7, and the noble gases 8 (helium 2). Carbon's four valence electrons are why it forms four bonds and builds diamond, graphite, and every organic molecule. This count sets an atom's whole bonding personality: how many bonds it wants, and whether it prefers to give electrons away, take them, or share.
Valence electrons decide which kind of bond forms. Metals hold their few valence electrons loosely and let them roam (the metallic bond and its electron sea); non-metals grip theirs tightly and either share them (covalent) or snatch a neighbour's (ionic). So counting valence electrons is often the first move in predicting a material's structure and behaviour, whether it is conductor or insulator, hard or soft, high or low melting point.
Silicon (group 14) has 4 valence electrons, so each silicon atom forms 4 covalent bonds to four neighbours, building the open diamond-cubic structure that underlies every computer chip. Change to phosphorus (group 15, 5 valence electrons) and the one extra electron is what makes silicon into an n-type semiconductor when phosphorus is added.
Count the outer electrons and you can often guess how an atom will bond.
Valence is fuzzier for the transition metals (iron, copper, and their neighbours), where d-shell electrons can also join in, giving several possible valences: iron is famously both Fe^2+ and Fe^3+. The simple group-number rule works cleanly mainly for the main-group elements.