what do you mean by shared electron and unshared electrons

Chemical bond which takes place between similar atoms,cannot takes place by gaining or loosing of electrons.In such cases the bond is formed by mutual shairing of electrons.Such type of bond or force which binds atoms of same or different elements by mutual shairing of electrons is called as covalent bond.
In covalent bond the pairs of electrons which is responsible for the formation of bond is also called as shaired pair of electrons or also called as bond pair of electrons.For ex- in Cl₂ molecule ,each Cl atom has 7e-s in valence shell and needs 1 more e- to complete the octet,so both atoms contribute 1e- each to share 2e-s.These are shaired pair of electrons and bond is represented by putting a line(--) between the atoms.
The valence electrons not involved in shairing are called as non-bonding or unshaired electrons or lone pairs.Each Cl atoms has 3 unshaired pairs.