What is spectrochemical series and what is its importance?

Hint: In coordination chemistry a ligand can be defined as an ion or molecule that binds with a central metal atom to form a coordination complex. The bonding with the metal generally involves the donation of one or more electron pairs.

Complete answer:
Spectrochemical series was founded in 1938 which is a list of ligands according to their strength or we can say a list of metal ions based on their oxidation number, group and identity. We know that in crystal field theory ligands modify the difference in energy between the d orbitals known as crystal field splitting.
Ligands are generally known as electron donors and metals are electron acceptors. Ligands are of two types: weak field ligands and strong field ligands. Whereas strong field ligands are those ligands which produce a large crystal field splitting which further leads for low spin value and on the other hand weak field ligands are those ligands which have low crystal field splitting and leads for high value.
A series which arrange the ligands from large splitting value to small splitting value is known as spectrochemical series which can be shown as:

It is important to know easily about weak field and strong field ligands.

Note:
Nature of metal to ligand bonding generally ranges from covalent to ionic and also ligand bond order can also range from 1-3. Ligands can be classified into different categories on the basis of charge, size, identity of coordinating atoms, denticity or hapticipity.