Today marks the vernal equinox, often described (in the Northern Hemisphere) as the first day of spring. The word equinox refers to “equal night”: for a lot of Earth, today marks roughly 12 hours of day and 12 hours of night. In the Northern Hemisphere, we’ll start seeing increasingly more than 12 hours of daylight from here until the summer solstice.
Contrary to common belief, the seasons on Earth are not due to how close we are to the Sun. If that were true, both hemispheres would experience summer at the same time! Instead, the seasons of Earth are because our axis of rotation is tilted compared to our orbit, so part of the year the North Pole is pointed more towards the Sun and the other part of the year the South Pole is pointed more towards the Sun. The equinoxes (vernal and autumnal) mark the transitions between these two: for today, neither pole is pointed more towards the Sun than the other. Before today, the South Pole was more towards the Sun; after today, the North Pole will have that honor.