Blood moon is just a total lunar eclipse. What happens when there is a lunar eclipse is that the earth gets in between the sun and the moon, therefore shadowing the moon, causing the eclipse. It turns the moon into a reddish to orange color.
There are 4 of these blood moons happening in 2014 and 2015, all of them falling on Jewish feast days. This is call a Tetrad.
A lot of interest is coming from this because the bible does mention about the moon turning to blood. In addition, the fact that they will be falling on Jewish feast days makes it seem like a sign for something brewing in the skies.
Mark Hitchcock puts all these into a historical and biblical perspective in his book, Blood Moons Rising.