An equinox is a point in the Earth’s orbit when day and night are both exactly the same length.  There are two equinoxes (or equinoctes if you want to be excessively correct) each year, but they also mark the change of the seasons between warm and cold.  The Earth’s axis is tilted, so that for half of each year, one hemisphere is tilted towards the Sun, so that it gets more heat and light from the Sun that the other.  The days are longer, and the weather is warmer.  As the Earth moves in its orbit, the two hemispheres slowly take turns moving closer and then further from the Sun.  The brief moment when they’re swapping place and are both at the same distance from the Sun happens at equinox.  The equinoxes mark the beginning of astronomical spring and autumn, although in reality we usually notice the seasons begin to change at least a month before and after those dates.  Incidentally, the extreme points when a hemisphere is deepest in shadow or closest to the Sun is called a Solstice, and marks the beginning of astronomical summer or winter.