She meant that the human race has acquired the ability to alter the nature of the Earth.
Examples of a few neat things that we have the power to do:
-- Spread diseases to other continents from where they are.
-- Hunt and kill entire species of animals to extinction.
-- Eradicate the habitat of whole species so that they can't survive.
-- Poison the air and water over an area large enough so that entire species become extinct.
-- Poison the air well enough so that species of vegetation ... plants and trees ... are wiped out.
-- Extract so many members of a marine species from the ocean that the entire population dies, along with other different species that we're not even interested in but need the species that we wiped out, in order for THEM to live.
-- Add enough stuff to Earth's atmosphere to change the climate distribution on the planet, move the hot and cold places around, move the wet and dry places around, and force the migration of almost every living thing on Earth ... wiping many of them out.
It took us a while to learn how to do these things, but we are so smart that we were able to stick to it until we succeeded.
By the way ... you really ought to go back and read this answer again, and in the places where I wrote 'species', remember that human beings (us) are one of the Earth's species.
