The meaning of Green Tara is goddess of compassion, and why she sits with one foot extended is because she is ready to come to the aid of the suffering in an instant. She is a goddess of compassionate action and protection.
It's the Tibetan syllable TAM, the seed syllable of the female buddha Tara (sanskrit) or Dölma (tibetan), which translates as Liberatress. As it is white here, it stands for the White Tara, which especially gives the blessings for a long life:
Tara - A female Buddha and meditational deity, is arguably the most popular goddess in the Buddhist pantheon. She is considered to be the goddess of universal compassion who represents virtuous and enlightened activity.
According to Buddhist tradition, Tara was born out of the tears of compassion of the bodhisattva Avalokiteshvara. It is said that he wept as he looked upon the world of suffering beings, and his tears formed a lake in which a lotus sprung up.