The "turn" is a musical ornament; as MTT says, it's a kind of musical pirouette. It features prominently in Mahler's early music, and he returns to it in his last works.
The turn had a long and deep significance for Mahler that dates back to his earliest musical memories. In the synagogue, he would have heard the turn frequently as a ornament in the chanting of the Torah: for instance, in the cantillation called gershayim. He would probably have even sung it while reading the portion assigned to him for his bar mitzvah.
In Mahler’s later music, the turn, once a simple item of musical punctuation, comes to occupy a place of particular emotional significance. Here it is at the beginning of the last movement of The Song of the Earth (Der Abschied).
Play the extended video on the right to explore Mahler’s highly idiosyncratic use of the turn in the Ninth Symphony.