La tua cantante is the Italian expression for a human whose blood particularly appeals to a certain vampire in an extreme sense. In English, it means "your singer", but is usually shortened to "singer" whenever someone refers to her/him.
The smell of each human is different, and certain humans can smell more appetizing than is usual to vampires. The more appealing any human's scent and taste is to a vampire, the more difficult it will be for that vampire to leave the human alive.
Infrequently, a specific human will smell nearly irresistible to a specific vampire. "Singers" are individual phenomena; a person whose blood sings for one specific vampire will not have the same effect on all vampires, though they may still find the human mouthwatering (As Laurent said to Bella when she was in the meadow that she and Edward used to hang out before he left Forks). While there are some humans whose scents are more appealing to vampires in general, that appeal does not reach the level of a singer.
Singers are considered by most vampires to be a great find, the drinking of whose blood is an experience to be savored.
Nahuel's mother, Pire, was also Joham's singer, though not quite as potent as Bella to Edward, he maintained enough control to impregnate her. The blood of singers is described by Edward as being said vampire's "own personal brand of heroin".