Metaphors are comparisons that don't use the words like or as. A good metaphor compares two unlike things in ways that are unusual but that strike readers as true.
However, some metaphors become so overused that they lose their freshness and meaning. Orwell refers to these as "dying" metaphors. He also calls these stale phrases "worn-out" metaphors. He gives a list of many examples that were current in his time but are no longer often used, though some that he mentions, such as "run roughshod over" and "axe to grind," are still used today.
Though not exactly a metaphor, an example of a stale and overused phrase society has encountered during the COVID-19 pandemic is "new normal." The juxtaposition of the terms new and normal, which seem to be opposites and have a pleasing alliteration, might have been fresh when the world was first adjusting to lockdowns and masks; but by now, the term has become overused and, to some, irritating.
Orwell objects to worn-out metaphors because they are used as a substitute for thought. Writers have heard these phrases so many times that they simply plug them into their writing without stopping to ponder what they mean or to consider if there is a more vivid or precise way to convey what they want to say. Likewise, readers encounter them and are so used to them that their meaning is less clear. Thus, language becomes devalued.