This is "A Concert" by Lorenzo Costa. Painted in the Italian north around 1488-90, it's an early example of a type of image that would become incredibly popular in northern Italy and Venice during the sixteenth century.
The painting depicts a group of musicians engrossed in music-making. The young woman is singing while keeping time with her fingers on the marble ledge and her companion's shoulder. He's deep in concentration, his eyes lowered to an open book containing the musical composition - its notes have only been roughly sketched in.
The musician behind him appears to be straining to reach or hold a note, and he too is tapping time on the ledge. The trio's concentration and exertion are tangible, enhanced by the deep black background. Their music might be a complex one, possibly a frottola - a three-part song written for tenor, soprano, and bass accompanied by a lute.
The woman's dress adds luxurious embellishment to the otherwise stark image. The inclusion of instruments not only suggests their musical skill but also creates depth in the painting. Some have suggested that this could be an allegorical representation of Harmony or the art of music, but it's likely that the picture was made to entertain a wealthy patron who enjoyed singing from a book - a developing trend in northern Italy at the time.
A Concert hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.