Well, let's go back to the actual empirical data

many years ago, when measuring volumes and weights and other such "gravimetric" properties, people noticed that there were some compounds where the elements involved didn't seem to be using their full valency.

and also, when doing calorimetric and thermodynamic measurements, some people noticed that the bonds in certain compounds seemed to contain waaaay more energy than 'normal'

Examples might be things like Nitrogen gas (only two N atoms in the molecule, but a very strong bond between them)

or ethene (as opposed to ethane) - less hydrogens in ethene, stronger carbon to carbon bond....

to explain things like this, they developed the notion of "multiple bonding" - according to the Lewis bonding model,
we say that the covalent bond consists of TWO or THREE pairs of electrons, rather than just one.

in Molecular Orbital theory, we say there's a net excess of TWO or THREE pairs of electrons in bonding orbitals.