Economists are frequently asked for advice from governments on policy matters. What should the tax rates be? Should we build a road or build more public transport? How can we reduce the amount of pollution in our rivers without losing lots of jobs?
Often these decisions involve tradeoffs: some people will benefit, and others will do worse, as a result of any decision the government makes. And it is difficult to say whose interests should be favoured when a tradeoff like this occurs. So economists often try to avoid answering a question in those terms. But they have another concept which can give useful advice that is less controversial: they can advise governments how to avoid inefficiency.
The intuitive idea behind efficiency is pretty straightforward. If you are doing something inefficiently, you are wasting an opportunity. You could get more benefits from the same resources. So inefficiency seems uncontroversially a bad thing.
But the sorts of definitions that economists use for efficiency are more technical than this, and it can be hard to understand what they have to do with waste. This video explains the connection.