Ever read Da Vinci Code?

If you did, the book refers to the holy grail thousands of times. Originally, the Holy Grail was the dish, plate, or cup used by Jesus at the Last Supper, said to possess miraculous powers (For more detail see http://en.wikipedia.org/wiki/Holy_Grail).

What it meant in your teacher's comment depends on the context. Generally speaking, it's okay to say holy grail 延伸意义是终极目标或 top prize 的意思.