“The symbol above represents extinction. It was created by London artist ESP in 2011. The circle signifies the planet, while the hourglass inside serves as a warning that time is rapidly running out for many species. The world is currently undergoing a mass extinction event, and this symbol is intended to help raise awareness of the urgent need for change in order to address this crisis. Estimates are that somewhere between 30,000 and 140,000 species are becoming extinct every year in what scientists have named the Holocene, or Sixth Mass Extinction. This ongoing process of destruction is being caused by the impact of human activity. Such a catastrophic loss of biodiversity is highly likely to cause widespread ecosystem collapse and consequently render the planet uninhabitable for humans.”Taken from extinctionsymbol.info. This paragraph is worth reading more than once.
More information on the mass extinction currently taking place: http://www.biologicaldiversity.org/programs/biodiversity/elements_of_biodiversity/extinction_crisis/
See also: The story behind the Extinction Rebellion symbol at climate protests at Grist.org.