Redox is the term used for reactions which involve both reduction and oxidation. Oxidation is loss of electrons (OIL) Reduction is gain of electrons (RIG) Oxidising agents are reduced - they gain electrons. Reducing agents are oxidised - they lose electrons.
More reactive metal can replace a less reactive metal. e.g. Fe + CuSO₄ --> FeSO₄ + Cu Sulphate ions in most redox reactions are spectator ions. Fe to FeSO₄ - oxidation so loss of electrons. Fe oxidation state from 0 to +6 so Fe --> Fe²⁺ + 2e⁻ CuSO₄ to Cu - reduction so gain of electrons. Cu oxidation state from +6 to 0 so Cu²⁺ + 2e⁻ --> Cu