Yarr! is like saying yeah! It can convey a sense of loss, a feeling of resination, and can even confirm agreement, but more appropriately it's just yeah.
An interjection. Can be used in nearly any sense, to convey any manner of emotion or simply waste breath and fill up blank space in a conversation. Derives its origin from the 'yarr' used by pirates during the Golden Age of Piracy during the 1500s to 1700s and appropriated from there.