The poisoned apple is sometimes used to symbolize what the Queen intended to do to Snow White. In folktales and mythologies, apple symbolize immortality, man's fall to sin, temptation, and knowledge. It was featured in many fairy tales aside from Snow White and the Seven Dwarfs.
The apple, on the other hand, is a little more tricky. In the Grimm tale, the apple is only half poisoned. Originally it is a white apple, but the Queen poisons half of it, which turns it into an alluring red. When feeding the apple to Snow White, she keeps the white half for herself and gives the red half to Snow White. Red and women becomes a symbolism of mensuration, the physical transition into womanhood. But the apple also holds other loaded symbolism: Apple of Eden, Apple of Discord. So one way or another, the apple is supposed to represent Snow White’s loss of innocence, something else that occurs with the transition into womanhood.