A solar eclipse happens when the Moon passes in front of the Sun and casts a shadow across Earth. It's also known as an occultation. The reason solar eclipses happen is that the distance between the Sun and the Earth is about 400 times the Moon's distance from the Sun and the Sun's diameter is approximately 400 times larger than the Moon's.
What this means is that the Sun and the Moon both have a very similar size when viewed from Earth, so when the Moon passes in front of the Sun, it blocks the light from reaching Earth.
Many people had some interesting ideas about what the eclipse was. Here are some stories about them.