Metallic bonding is the electrostatic interaction between metal cations and their delocalised electrons.
Metal atoms have a tendency to lose electrons so that they have a full set of valence electrons on their outer shell. In metallic bonding, the metal atoms lose their valence electrons to a delocalised "sea" of free flowing electrons surrounding the individual atoms.
The atoms become positively charged ions (cations) as their electrons are lost and delocalised. The electrons are no longer confined to a single atom and instead are shared among the cations.
The electrostatic forces between the positively charged ions and the negatively charged sea of electrons hold the ions in a lattice structure. These attractive forces help balance the repulsive forces between positive ions.
Metals are generally good conductors of heat and electricity because the electrons are free to move around. Electrons in the delocalised sea are mobile and can conduct current, unlike the electrons locked in covalent bonds.