Equinox translates literally to equal night.
On Sunday, March 20, 2016, the sun crosses directly over the Earth's equator. That moment is known as the vernal equinox in the Northern Hemisphere announcing the arrival of spring and the autumnal equinox in the Southern Hemisphere announcing the arrival of fall. A second equinox will occur on September 22, 2016.
The fact that the Earth has distinctive seasons is due to the 23.4 degree tilt of the Earth's axis. The Earth receives more sunlight (longer daylight hours) in the summer and less sunlight (fewer daylight hours) in the winter. The tilt of the axis makes the seasons opposite in the Northern and Southern Hemisphere. At the north pole summer gives six months of daylight while at the same time the south pole is experiencing six months of darkness. The closer you are to the equator, the daily hours of daylight and darkness become more equal.
The fall and spring equinoxes are the only two times during the year when the sun rises due east and sets due west. Modern astronomy aside, people have recognized the astronomical connection to the season changes for thousands of years. The ancients of various civilizations all over the world built structures that illustrate this—temples dedicated to their various gods that modern man recognize as observatories. Not only the spring and fall equinox days, but also the summer and winter solstice days.
I think it's also interesting to note a connection between the spring equinox and Groundhog Day (another holiday derived from the practices and celebrations of the ancients). If the groundhog sees his shadow on February 2, we have six more weeks of winter. And by coincidence that six weeks takes us to within a few days of the spring equinox.
A little bit of equinox trivia: According to folklore, you can stand a raw egg on its end on the equinox. One spring, a few minutes before the vernal equinox, twenty-four almanac editors tested the theory. For a full work day, seventeen out of twenty-four eggs stood up on the large end. Then three days following the equinox, they tried the same test again. And guess what? The results were similar. Perhaps the second test was still too close to the equinox? :)