The Earth orbits the Sun. However, the angle it is orbiting at is tilted. Think of it like a spinning top that is leaning over slightly. The tilt is actually 23.5 degrees and this is what gives the Earth its seasons. The tilt always stays at 23.5 degrees relative to the plane of the solar system but as the Earth orbits the Sun, the tilt relative to the Sun changes. At the solstice in June, the Northern Hemisphere of the Earth is tilted towards the Sun. At the December solstice, the Southern Hemisphere is tilted towards the Sun.
Twice per year the orbit of the Earth and the Sun line up so that although the Earth is still tilted, the tilt relative to the Sun is zero. The means that from the equator the Sun appears directly overhead. It also means that day and night are the same length and the Sun rises due east and sets due west. This time is known as the equinox and happens in around the 20th March and 22nd September each year. In the Northern Hemisphere, the March equinox is called the spring equinox or the vernal equinox while the September equinox is called the autumn equinox. The names are reversed in the Southern Hemisphere. The change in orientation of the Earth between equinox and solstice is shown in images below. The name equinox comes from the Latin words roughly translating to “equal night”.
Equinoxes have always had a significance in human culture. The equinoxes are celebrated throughout the world. There is evidence of many ancient sites such as tombs being aligned to the rising and setting of the Sun on the days of the equinoxes. Some calendars, such as the Hindu calendar marks the spring equinox as the start of a new year. Many ancient festivals are also linked to the equinoxes. Interestingly, communications with geostationary satellites can be disrupted around the time of equinox as the sun appears directly behind them in the sky. This means that there can be a lot of noise from the Sun, drowning out the satellite signal.