Nitrogen fixation is the process in which procaryotes take the atmospheric nitrogen (N₂) and use it to produce ammonia (NH₃). Ammonia can be used by plants as a nutrient that helps them grow properly.
The role the process plays is very important because it transforms the nitrogen of the air into usable nitrogen for the plants.
Nitrogen is a very important nutrient in nature and agriculture when in the absence or low in supply, the plants can't grow healthy.