Apple pie is a fruit pie made out of pastry and apples, usually cooking apples such as the Bramley. The first record we have of apple pies dates from the 14th century. When English settlers first came to America, they brought apple trees with them, and along with the apple tree, recipes for making apple pies.  In the English colonies, most apples that were grown were made into cider, and meat was used in pies when apples where unavailable. American apple pie recipes date back to the 1700's, and have since risen greatly in popularity with Americans.
Because of their popularity in the United States, apple pies are often associated with things uniquely American. For example, something can be said to be as "American as apple pie".