What is the meaning of The Sixth Extinction?

The title of Elizabeth Kolbert's book The Sixth Extinction refers to the phenomenon also known as the “Anthropocene extinction” or “Holocene extinction,” an ongoing, sixth mass extinction event occurring after the Ordovician–Silurian extinction events, the Late Devonian extinction, the Permian–Triassic extinction event, the Triassic–Jurassic extinction event, and the Cretaceous–Paleogene extinction event.
The term “Anthropocene extinction” has been proposed as an extension of the Holocene extinction to emphasize the impact of the spread of human civilization on the environment. The name “Anthropocene” is a combination of anthropo-, from anthropos, meaning "human," and -cene, from kainos, meaning "new" or "recent."
Though there is some debate over the claim the that the Anthropocene extinction constitutes a new extinction event, polls of biologists indicate that most consider the Anthropocene extinction real. Numerous studies and papers have suggested that upwards of 70% of biologists acknowledge the existence of the Anthropocene extinction.
There is little consensus as to when the Anthropocene extinction began. Some have suggested that the Anthropocene extinction may have begun as early as when the first modern humans spread out of Africa between 100,000 and 200,000 years ago. Others have fixed the start point as late as the start of the industrial revolution in 1780.
Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time. One calculation predicted that if the current rate of human disruption of the biosphere continues, one-half of Earth's higher lifeforms will be extinct by the year 2100. At the present time, the rate of extinction of species is estimated at 100 to 1,000 times higher than the historically typical rate of extinction of life forms on the planet. This means that the rate of extinction in the Anthropocene extinction is 10 to 100 times higher than any of the previous mass extinctions in the history of the planet.
Human contributions to the extinction include deforestation, hunting, pollution, and the introduction of non-native species to different ecosystems. Humans have also spread infectious diseases through livestock and crops.
Additionally, climate change is one main theory behind the magnitude of the Anthropocene extinction. Though some scientists have argued that the Earth is going through a natural, historically precedented, change in climate, most have emphasized the impact of human civilization on the environment. Human activity since the Industrial Revolution has increased the amount of greenhouse gases, such as CO2 and methane, in the atmosphere. Studies have estimated these increases in CO2 and methane to be anywhere from 36% to 148% since the beginning of the industrial revolution. CO2 samples from ice cores indicate that these levels are higher than any time in the last 800,000 years.
There is significant evidence that the impact of human civilization on the environment constitutes a new, ongoing extinction event, possibly the biggest in history.
https://time.com/3035872/sixth-great-extinction/

https://www.cnn.com/2017/07/11/world/sutter-mass-extinction-ceballos-study/index.html

https://www.newyorker.com/magazine/2009/05/25/the-sixth-extinction