As the Earth rotates on its axis and revolves around the Sun, its axis is always pointing in the same direction in space. Right now, it happens to be pointed toward a star called Polaris (the North Star) in the constellation of Ursa Minor. Because the Earth's axis wobbles a bit, it hasn't always pointed in that same direction - it was different a few thousand years ago (the ancient Egyptians used the star Thuban in the constellation of Draco as a pole star) and will be different a few thousand years from now.
So, as planet Earth orbit the star that is our sun, its Northern and Southern Poles are sometimes tilted toward the Sun and somtimes tilted away from the Sun. Here in the Northern Hemisphere this year, we're tilted the most toward the Sun on June 21 - the Summer Solstice - and tilted the most away from the Sun on December 22 (Eastern time) - the Winter Solstice. The equinoxes are the points between the solstices on March 20 (Vernal or Spring Equinox) and September 23 (Autumnal or Fall Equinox) when we're neither pointed toward nor away from the Sun.
Again, at my latitude, on the Summer Solstice, since we're tilted towards the Sun, the sunlight can wrap more than halfway around the Earth and days are around 15 hours long, on the Winter Solstice, since we're titlted away from the Sun, the sunlight can't even make it halfway around the Earth and days are only 9 hours long. Today, on the Equinox, the Sun wraps halfway around the Earth and days are 12 hours long (sunrise is around 7 am and sunset around 7 pm EDT).
For winter's rains and ruins are over,
And all the season of snows and sins;
The days dividing lover and lover,
The light that loses, the night that wins;
And time remembered is grief forgotten,
And frosts are slain and flowers begotten,
And in green underwood and cover
Blossom by blossom the spring begins.
Atalanta in Calydon
Algernon Charles Swinburne (1837-1909)