Germinal looks backward and forward, in both ways with hope, and it acts as a warning to the upper classes to get their act together and start treating the laboring poor with more decency. Looking back, the word invokes the world of the French Republic, started by a revolution brought on by hunger and want that toppled the king and the aristocracy. The new government went so far as to rename the months of the year, calling the first month in spring, starting at about March 20, Germinal, a time of seeding or new birth, following by the month Floreal or flowering. Although this calendar was abandoned in 1805, it was brought back in 1871 by the Paris Commune, a radical government that ruled France for 18 days. That memory, not so long before the 1885 publication of Germinal, would still be vivid in many people's memories. So the title functions as a warning—the seeds of a revolution are again being planted in the hearts and minds of the suffering miners and, by extension, all of the suffering laboring classes.
The word "germinal" means germinating or slowly growing and is related to the earth's fecundity or amazing ability to propagate itself and burst into new life after lying dead and dormant through the winter months. Notably too, in the novel, the poor miners have literally next to nothing and are almost always hungry because they can't even earn enough money slaving in the mines to buy food. They certainly can't afford any entertainment, but sex is free and it takes their minds off their hunger. It's also pleasurable, so it's what the miners do when they aren't working. They are constantly making love, and of course, in humans, this germinates new life. Their fertility and fecundity are put in marked contrast to the sterility and lack of sex and vitality in the mine owners and managers. The upper classes live amid many luxuries, with carriages and silk clothes, fine furniture and all the food they could ever eat, but they are sexually repressed and lifeless people to the point that the reader almost feels sorry for them and their barren lives.
The vitality, sexuality, the strike, and the ideas germinating—seeded, planted—in the miners are all a warning to the ruling class that a new day is dawning; the revolution is coming. This is the meaning of the title, as summed up in the following passage:
Men were springing forth, a black avenging army, germinating slowly in the furrows, growing towards the harvests of the next century, and their germination would soon overturn the earth.