A "blue moon" is a second full moon in the same month. Usually, there is just one full moon a month. But due to differences between the length of calendar months and the length of the lunar month, a second full moon can occur in the same month. This happens once every 2.7 years.
For occurrences until 2015 see wikipedia:en:Blue_moon#Blue_moons_between_2009_and_2015.