Molecular orbitals are formed from the combinations of atomic orbitals.

Since orbitals are wavefunctions, they can combine either constructively to form bonding molecular orbitals or destructively to form antibonding molecular orbitals

Why do we need Molecular Orbital theory?

Valence bond theory generally fails to explain the bonding in simple molecules. On the other hand, molecular orbital theory is better approach for the molecules those are having extended p systems.
With the help of molecular orbital one can understand the electronic transitions in molecules.