Confusion in the line written in my book
Ok let me explain you expecting you are from a cbse affiliated school.

Group oxidation state = (group number - 10)

This will give you the group oxidation state of that group. Group oxidation state is the maximum oxidation state shown by the elements of the group. For example in 15th group, the group oxidation state = (15-10) =5
That means the maximum oxidation state which the group 15 elements would show is 5.
Generally moving down any p-bock group, the tendency of occurence of the maximum oxidation state (i.e. +5 for group 15 elements, +3 for group 13 elements) decreases. Rather the most occuring oxidation state for heavier elements is the oxidation state 2 units less that the maximum/group oxidation state. The reason behind this is intervening of d and f orbitals for heavier elements i.e. in one word we say the inert pair effect. 

Now what is inert pair effect? As we move down the group, gradually for heavier elements d and f orbitals start filling up. The shielding by electrons in d and f orbitals is very poor due to which moving down the group the nuclear charge increases, but there is not much increase in shielding due to which effective nuclear charge becomes effective and hence its not possible for the heavier elements to attain the maximum oxidation state because to attain the maximum oxidation state, they have to loose the more number of electrons. But that is not possible for heavier elements because the nuclear charge is now effective. It wont allow loosing much of the electrons. Hence the maximum oxidation state/ group oxidation state is not achievable for the heavier elements. 

What they can achieve is two units less than that maximum oxidation state and that also makes sense. For example in case of group 15 elements, the maximum oxidation/ group oxidation state i.e +5 becomes unstable for heavier elements but +3 state (i.e. two units less than the maximum oxidation state/ group oxidation state) becomes increasingly stable for the heavier elements. Now you might think why only two units less than the maximum oxidation state stable for the heavier elements? Why not 3 units less? Because this is a fact i.e. because we have found the more observing/occuring oxidation state for p block elements is two units less than their corresponding group oxidation state. Now the statement was written because it was observed. 

Negative oxidation state for heavier elements is also not achievable because down the group the shell size is large. Though I said effective nuclear charge is bit effective for heavier elements, but that doesn't mean it is stronger enough to gain an electron. It is just strong to get hold of some valence electrons i.e. not allowing loosing much of the electrons. But that strength is not that strong to gain an electron. Hence negative oxidation is not shown for heavier elements. 

So basically stable oxidation state for heavier elements is 2 units less that the maximum oxidation state shown by that corresponding group.