To put it in simple terms: at the equinox, the Earth's axis is halfway in its apparent movement from pointing away from the Sun (northern winter solstice) to pointing towards the Sun (northern summer solstice).

At each equinox, the Earth's axis is exactly at right angles to where the Sun is, which means the Sun rises due east and sets due west, and day and night are of equal length.

The difference between the two is that after the Spring (or Vernal) Equinox – in late March for the northern hemisphere – the days are longer than the nights, while after the Autumnal Equinox – in late September – the nights are longer than the days.