It's an eight day long Jewish holiday celebrating the rededication of the Temple in Jerusalem at the time of the Maccabean Revolt against the Seleucid Empire of the 2nd century BCE. Basically, the temple was seized by the army of a Syrian-Greek ruler, practicing Judaism was made an offense punishable by death, and they tried to bully the Jews into practicing their faith instead. After the Jews were able to overthrow them, they had to clean up and rededicate the temple, but could only find enough oil to light the menorah for one day. The story goes that the oil actually lasted for eight nights. The holiday is supposed to celebrate the miracle and the victory over their oppressors.

It's not a very important holiday in the Jewish calendar, but it has an interesting history and has become more popular because it usually falls fairly close to Christmas.