Hi ,
The oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by integers, which can be positive, negative, or zero. In some cases the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe3O4). The highest oxidation state is +8 in the tetroxides of xenon, ruthenium and osmium, while the lowest is −4 for some elements in the carbon group.

The increase in oxidation state of an atom through a chemical reaction is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons, a net gain in electrons being a reduction and a net loss of electrons being an oxidation. For pure elements, the oxidation state is zero.
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Oxidation State
 Oxidation state is a number assigned to an element in a compound according to some rules. This number enable us to describe oxidation-reduction reactions, and  balancing redox chemical reactions. You are learning the skill to assign oxidation states (or oxidation numbers) to a variety of compounds and ions. 
When an oxidation number is assigned to the element, it does not imply that the element in the compound acquires this as a charge, but rather that it is a convenient number to use for balancing chemical reactions. The guidelines for assigning oxidation states (numbers) are given below:
  
 The oxidation state of any element such as Fe, H2, O2, P4, S8 is zero (0). 
 The oxidation state of oxygen in its compounds is -2, except for peroxides like H2O2, and Na2O2, in which the oxidation state for O is -1. 
 The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, etc., in which the oxidation state for H is -1. 
 The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion. 
 The following elements usually have the same oxidation states in their compounds:  

+1 for alkali metals - Li, Na, K, Rb, Cs; 
+2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba; 
-1 for halogens except when they form compounds with oxygen or one another;