explain very briefly the process of biological nitrogen fixation in the nodule

Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation. Certain forms of bacteria are able to convert atmospheric nitrogen into usable forms such as nitrates and nitrites. Such nitrogen fixing bacteria like Rhizobium are commonly found in the roots of legumes inside special structures called root nodules.

They are able to fix nitrogen because of the presence of  exclusive enzyme nitrogenase in them. 
N   ≡   N   NH₃