This piece comes from an Xerxes - opera seria in three acts. The opening aria, "Ombra mai fu", sung by Xerxes to a tree, is set to one of Handel's best-known melodies, and is often played in an orchestral arrangement, known as Handel's "Largo".
It is often performed at solemn occasions such as funerals and weddings. It was arranged for all sorts of instruments and voices.