Nitrogen Covalent Bonds
Since you have zero knowledge of chemistry, I will keep explanation as simple as possible. I have to emphasize that this answer is not fully correct and can be misleading. It is suited just for interested layman, but not someone trying to grasp deeper concepts of chemistry.  

Nitrogen atom has 5 electrons in its outer shell. The most stable electronic configuration, by octet rule, corresponds to 8 electrons in outer shell. Nitrogen needs three electrons to make that shell full. It can achieve that configuration by bonding with another nitrogen. One nitrogen has his own 5 electrons and another lends him his 3 - now first nitrogen has 8 electrons, e.g. full shell. First nitrogen lends his 3 electrons to second nitrogen which also had 5 electrons, so now both of them have closed shell. Since both of them lended three electrons, and accepted three - they have three shared electron pairs, which corresponds to triple covalent bond. Closed shell configuration, which they both now achieved, corresponds to a state of the lowest energy. Since everything in nature strives to a state of lowest energy, you can see why $\ce{N}$ atoms prefer to bond, instead of being each on it's own. The molecule is inert, because it achieved what it strived to: energy minimum. By adding another (third) nitrogen in a game, you would necessary have to "break" closed shell configuration, which necessary leads to increase of energy, which is not favorable. That's the reason why you can't create another bond and keep inertness of $\ce{N2}$ molecule.