Atoms in a metal are arranged in a lattice formation. What is a lattice? Imagine a grid (squares on a sheet of graph paper for example) where there is a metal atom in each square of the grid. This is called a metal lattice.
Metal atoms like to lose electrons. Why? Because they want to have a full outer shell of electrons. How many electrons do they lose? It depends on the metal. Different metals have different numbers of electrons in their outer shells. For example, sodium has 1 electron in its outer shell so it only loses 1 electron; aluminium has 3 electrons in its outer shell so it loses 3 electrons.
The outer shell electrons of each atom in a metal lattice can therefore move around the lattice. We call these electrons delocalised or free electrons. (It is these free electrons which make metals good conductors of both heat and electricity.) Since the outer shell electrons have left their atoms, we are left with a lattice of positively charged metal ions and a “sea” of negatively charged free electrons. Opposite charges attract each other, so the positive metal ions are attracted to the negative free electrons. This electrostatic interaction is metallic bonding.
The sea of free electrons is like a glue that holds all the metal ions together. This ‘glue’ is very strong which is why metals have very high melting points and boiling points.