What "fixing" nitrogen means it taking the–very unreactive–dinitrogen molecule which is the major component of air and turning it into something that is more reactive and can be used by plants to make, well, more plants (since nitrogen is an essential part of most important biological molecules).
Not every plant can do this (which leads to important discoveries in agriculture like crop rotation). The plants that can fix nitrogen don't actually do it themselves but use symbiotic microorganisms (which, in the case of legumes, live in the plants' roots). These microorganisms use a series of nitrogenase enzymes to do the conversion from the atmospheric nitrogen. Until the 19th century this was the major source of nitrogen for biological systems and it severely limited agricultural yields (there just isn't enough animal poop on the planet and even that, ultimately, is derived from nitrogen fixed by plants).
In the early 20th century a german chemist, Haber, worked out how to do it with a chemical catalyst based on iron and some expensive industrial plant (the other sort of plant, pipes and machinery, not the green photosynthesising kind). The process was improved and commercialised by BASF where Haber worked with Bosch (both got Nobel prizes for their work).
The process turns nitrogen into ammonia which is still one of the major bulk products of the chemical industry and can be turned into many other useful things. Initially the goal was fertiliser. For the first time agricultural yields could be greatly improved without relying on crop rotation or a supply of other existing nitrogen compounds. Of course ammonia is also useful for making nitric acid, essential in explosives, so the world immediately used it to kill many of the extra people it could now feed.
Even today the industrial process is important (we still need fertiliser and we still like to kill each other with bombs). But many useful chemical feedstocks for more benign compounds (plastics, drugs, cleaning agents...) are, ultimately, derived from the process.
So, for the production of ammonia, plants can do it with enzymes and we can do it with industrial catalysts. This is probably the biggest volume chemical where a natural enzyme can be replaced by an industrial catalyst and vice versa.