Apple seeds contain cyanide, a poison that is poisonous to humans. According to About.com, consuming the seeds of an apple, however, is not typically dangerous or lethal to people.
The CDC notes that cyanide is a chemical that is released from natural substances like plants. The cyanide in apples, apricots and peaches is released from chemicals that are metabolized into the poison. About.com states that the thick coating on an apple seed protects people from the trace amounts of the chemicals they contain. Even chewing and swallowing the seeds is not dangerous to humans because the seeds contain so little of the poison. petMD, however, warns that apple seeds and other fruit pits are toxic for most domestic animals.