From their Facebook page:
Be a part of the biggest grass-root effort in history by planting a seed/tree as a “give back” to Earth!
It's a Native American tradition that when you take something from the Earth, you must put something back. Earth Day 2015 will be a global "give back to Earth" event, as an "offering" for all the planet gives us. Our goal is to plant one billion seeds/trees. Please plant something that is organic, perennial and suitable to the growing conditions in your area. If the temperature is not conducive to planting on April 22nd, please start growing a plant indoors or commit to planting your tree once the outdoor temperature is warm enough. If you live in an apartment, please plant something in a container or donate a tree to a garden in your area. Please nurture your plantings, like children, until they can sustain themselves on their own.