What Itadakimatsu means?

I Just want to know when i can use it and why i should use it.

itadakimaSU i believe, not -matsu. It basically means "lets eat" or "bon apetite". From what I understand though, and I may be mistaken as this is secondhand knowledge from my native Japanese teacher, it is more or less a reflex to say this before a meal, and the meaning is not really considered (literally "I receive").

"Itadakimasu" is "I humbly and thankfully receive (this meal)," so it is something people say before a meal, and is meant to be an expression of gratitude to the ones who cooked it, to the food, to nature that provided the food to you :)

If you are in a formal situation, it is expected for you to say Itadakimasu before beginning to eat (just as a courtesy i think). So amongst the Japanese, do say it before eating to make a good impression of yourself :P