i think it often be used before have meals, mean "i will start eating", it shows respect to those who produce foodstuff such as farmers

"Itadaku" is an honorific/humble (I forget which) verb that means "to receive." So by saying "Itadakimasu" (the polite form of the verb), you say something like "I humbly receive/accept...." So when eating a meal, it's pretty much saying "I humbly accept this food." When ending a meal, you say "Gochisousama(deshita)."

tiance, Takashi, sakaya are all correct. I just want to add an explanation why we use the verb 'itadakimasu' for 'humbly receive'. 'Itadakimasu' literally meant "put something above one's head", although this usage is not used any more. But the noun 'Itadaki' is still used in the meaning of 'top or peak (of a mountain)'. In the ancient Japanese culture, when we receive something from higher person, we lower the head, and put the both hands up above the head, then recieve the thing on the hands. That's why 'itadakimasu' means 'receive something from higher person', then we use before eating. You can use this words when you recieve gifts too.

It translates literally as "I humbly receive". Remember that nobody says anything like this in English! If you think along the lines of the French saying "Buon appétit" you should get the idea :-)

tiance is correct. Just for your information, we have a song for "itadakimasu". Actually, this is called an "Obento (a lunch box)" song which has been sung in kidergartens in Japan for at least several decades. http://youtube.com/watch?v=KmQ97678SqM Here the kids say "Dad and mom, I'm going to eat delicious lunch. I will clear my plate. Now, let's eat! "