what is the exact meaning of oxidation state?
Oxidation state is a formal number ascribed to atoms, to tell if it has more or less electrons than in its natural state.

For single atoms it is fairly straightforward, as electrons can only be removed or added in discrete units. Therefore, an atom or ion has an oxidation state that equals it electric charge.

But electrons are not small particles, orbiting the molecule. In many ways an electron is better described as a cloud of negative charge hovering around the molecule and chemical bonds.

When different atoms are connected in a molecule, this cloud is not evenly distributed between the atoms. Example: In Carbonmonoxide (CO) the Oxygen attracts most of the electron-cloud. But not all, if so the molecule would part into ions.

In reality, the oxidation state of the atoms in CO should be calculated with decimals instead of whole numbers, but for practical reasons we ascribe the whole of an electron in a bond to one of the atoms.

In Carbonmonoxide the atoms are bonded by a double-bond (4 electrons) and if all of these “belongs” to oxygen it will have two more than the basic atom and thus the oxidation state is -2.