Pairs of electrons are shared in a bond. the actual number of pairs is called bond order

Covalent bonds hold non-metallic atoms together. These atoms have many electrons in their outermost level and have a tendency to gain electrons rather than to yield them.

In this case the bond is formed by sharing a pair of electrons between the two atoms, one from each atom. Then, this pair of shared electrons is common to the two atoms and holds them together, so that both atoms acquire more stability.