Modeh Ani is the first thing that a Jew should say when he wakes up in the morning. It is a prayer thanking God for having returned your soul to your body apon awakening. It should be followed by washing your hands.
This is done because, in Judaism, when you sleep it is almost as if you are dead. Your soul is not considered to be in your body. This mean that when you wake up you need to ritually cleanse yourself, as if you had been in contact with a dead body. In fact the Sages teach that sleep is a sixtieth of death.
Modeh Ani (lit. "I thank you") is also sung as a song praising God.
When its sung as a song the words "ma ma ma ma ma ma ma ma" and "da da da da da da da da da" often precede the verse. It also sounds really nice when sung in rounds.
Modeh ani lifanecha, melech hi v'kayam,
Shehechzarta bi nishmati b'chemla --
I give thanks to you, living and eternal king
For you have returned my soul to me with compassion --
Abundant is your faithfulness!
-- translation is basicly mine, I was taught it many years ago...
Modeh Ani can also be seen as a re-affirmation of the Jewish belief in Techiyas Ha'Meisim - The Resurrection of the Dead. In Judaism there is the belief that, after the messiah, comes then God will resurrect the righteous people of every generation. Just as God returns our soul to us each morning, so will he return life to the righteous.