"捨（す）てるにはまだもったいないよ（It is too soon to throw it away)".

You will hear the phrase if you live in Japan.
It is said that the word, Mottainai, has become widespread around the world thanks to Wangari Muta Maathai, who is an environment protection activist in Kenya.
Like Sushi and Tsunami, that are already recognized as English words, the Japanese word, Mottainai is also rapidly spreading around the world.
Although how the word is interpreted depends on the occasion, the phrase above means that something still has some worth, so you shouldn't waste it. 

When I was a little, I remember that "もったいないおばけ Mottainai Obake(ghost)" was a TV commercial in the 1980s created by the organization called AC Japan or Advertising Council Japan that produces official advertisements. 

Mottainai Obake represents that, when you waste goods, such as leaving food, the Mottainai Obake(ghost) will appear and punish you. When I was a child, my mother scolded me saying, "Don't waste food otherwise Mottainai Obake will come and get you!"
That was how we learned the spirit of Mottainai.

We Japanese have a tradition that a god lives in every single thing.
Mottainai Obake(ghost) is also one of the Gods who even lives in leftover food. 
We should remember the importance of nature because we believe it is equal to Gods and it is disrespectful to waste things.

We also believe that the behavior of spoiling food is disrespectful toward the hard work of the farmers. In other words, an attempt not to spoil goods(including foods) is paying respects to the farmers as well.

Nowadays, Japan is economically wealthy, so the importance of Mottainai might be less than before. However, I think we should remember to pay our respects toward the nature and goods again.
I hope the world will share the spirit of Mottainai.