The low reactivity of nitrogen molecules arises from the strong triple bond between nitrogen atoms.
The triple bond must be broken before nitrogen molecules can react; this is difficult due to its large bond enthalpy. Most reactions involving nitrogen are done at high temperatures, with catalysts.
Ammonia is nitrogen hydride, coming from the following reaction:
The lone pair of electrons on nitrogen is not involved in bonding in ammonia, so can form dative covalent bonds.