I am really confused at this part.The resonance as shown above causes the generation of a positive charge on nitrogen atom but the fact is that it's octet is still complete even after that resonance....what is the significance of this positive charge on nitrogen?What will it do to the nitrogen atom?
It doesn't result in the generation of a positive charge on the nitrogen. It results in the generation of a formal charge on the nitrogen. The formal charge is defined as
FC = V - N - B/2
with V being the number of valence electrons in the neutral, ground state atom, N being the number of valence electrons not involved in covalent bonds and B/2 being half the number of shared electrons in covalent bonds. Formal charge is the charge on the "ion" that is formed when it is assumed that all electrons "lost" are shared equally with the "ion" that captures them and it, in turn, lends an equal share of its bonding electrons.
At the left side nitrogen has 5 valence electrons. The pair is unshared and the other 3 participate in bonding. Were the bonds ionic and the electrons completely lost nitrogen would have a charge of +3. But we assume that the three that are lost are equally shared with the carbons that also contribute shares of their valence electrons so the formal charge on the nitrogen "ion" is 5 - 2 - 6/2 = 0.
Turning to the carbon which is marked in the drawing with a formal charge I get 4 - 1 - 3 = 0 so unless I'm not seeing something that's an error.
Now when the unshared pair moves over to form the second bond nitrogen loses two electrons but gets back a half share in each (and still has the half share in the pair from the original single bond) so FC = 5 - 0 - 4 = 1 For the carbon in this case: FC = 4 - 0 - 4 = 0