The music widget is one of the launcher's built-in widgets. It can be used to control media sessions on the device. The general usage is pretty self-explanatory: control the playback with the skip previous, skip next and toggle pause buttons. Tap on the album cover to open the current media player app. Long-press the album cover to open a player chooser.
It is backed by the media control integration which can be configured at Settings > Integrations > Media control.