Plato imagines someone moulding figures of all kinds in a lump of gold, and continually remoulding each and all into all the others. If someone points to one of these shaped pieces of gold and asks "what is it?" Three possible replies: by far the safest, "it is gold." 2, But never "triangle." 3. Unless they are willing to accept "such", that has some safety.
The most complete previous interpretation of this passage is by Harold Cherniss, AJP 75 (1954) 113-130. This paper discusses the difficulties in Cherniss' interpretation and offers alternatives.
Lee, Edward N., "On the "Gold-Example" in Plato's Timaeus (50a5-b5)" (1965). The Society for Ancient Greek Philosophy Newsletter. 34.