The reason for the title is two-fold. For one thing,he title refers to the French name of a month, Germinal, during which the Revolutionaries in 1795 demanded the government provide food. In one passage of the novel, such an event occurs. Second, the title refers to the French word germen (seed) and the constant hope the miners have for the growth of a better future.
In the story there is constant subtle and not so subtle reference to the hope of growth and change, just as a germinating seed begins to grow and change after a long time of being a dormant seed. The closing to the novel, for which it is quite well known, refers to the miners working down in the mine as germinating and rising up against unfair conditions after working in the harsh environment silently for so long.