A lunar eclipse is when the Sun, Earth, and Moon line up perfectly.
Because of this alignment, the moon seems to change colour and then 'disappear' because it isn't getting the amount of sunlight needed to make it shine in the night sky.
Basically, a lunar eclipse is when the moon falls completely behind Earth's shadow.
This was an event that historically, was unexplained. The Ancient Greeks were fighting the Syracuse Army when a lunar eclipse occurred.
The Greeks thought it was a message from the Gods telling them to stop, so they retreated, which eventually led to their defeat because Syracuse pushed forward, ignoring the lunar eclipse.
Another incident where a lunar eclipse altered history is when Christopher Columbus stopped at Jamaica while on his way to the new world.
He realized that the natives were plotting against him, as he had overstayed his welcome. Luckily, Columbus was aware that a Lunar Eclipse was going to happen within a few days.
Columbus told the natives that the moon would disappear if they did not continue feeding him!