Microsoft announced Xbox One Backward Compatibility at the E3 in 2015 and subsequently introduced the feature on November 12, starting with a catalogue of 104 games. More are added on an as-they-come basis.
By this term is meant the Xbox 360 games that are playable on the Xbox One. When using the same Xbox Live account on the Xbox One as on the Xbox 360, there is no need to purchase your owned compatible titles again. They are automatically added to your Xbox One game portfolio.
For a current list of games see: Xbox One Backward Compatibility GEEKList