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.
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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.
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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).
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Play the extended video on the right to explore Mahler’s highly idiosyncratic use of the turn in the Ninth Symphony.