According to MedicalNewsToday, apple seeds aren't actually as safe as you might think. They contain the compound amygdalin, which can be toxic. 

Amygdalin forms part of the apple seed's chemical defenses and, while it isn't harmful in itself, it can degrade into hydrogen cyanide when it's chewed or damaged. Cyanide is something that can be pretty dangerous — even deadly — if enough is consumed. Cyanide has been used in acts of murder in the past (via the Pittsburgh Post-Gazette) and interferes with the oxygen supply of cells. Apples aren't the only fruit whose seeds contain amygdalin either; the seeds of many fruits in the Rosaceae family contain amygdalin, from apricots and peaches to cherries and even almonds (via American Chemical Society). 

Fortunately, swallowing the occasional apple seed or two isn't going to harm you. It would take a lot of apple seeds to even make you feel unwell, let alone cause death. Per MedicalNewsToday, you'd need to eat between 83 and 500 apple seeds to develop acute cyanide poisoning, so as long as you don't make a habit of eating the seeds, you should be okay. Fortunately, the rest of the apple is fine to eat too — even if you want one each day.