not noticed or not used

It's important to apply research in a practical way and not leave it on the shelf. 重要的是，要把研究运用于实际当中，而不是束之高阁。

If someone, usually a woman, is on the shelf, she is not married and is considered too old for anyone to want to marry her.

In those days, if you hadn't married by the time you were 30, you were definitely on the shelf. 在那个时代，如果你到30岁还未结婚，那就肯定嫁不出去了。