on the shelf = not noticed or not used 被忽视的；被搁置的；废弃的。
be on the shelf 束之高阁，废弃的。
mainly UK informal（通常指女子）年龄太大而找不到对象的。

【例句】
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。

I screwed up the jar and put it back on the shelf.
我把罐子拧上盖儿，放回搁架。

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。