Can You Help Me Understand this Definition of Economic Efficiency Economic Efficiency:

"A broad term that implies an economic state in which every resource is optimally allocated to serve each person in the best way while minimizing waste and inefficiency. "

And here's the part I don't understand:

"When an economy is economically efficient, any changes made to assist one person would harm another."

Can someone please unpack that last part for me?  I'm not sure I understand it.
Suppose the economy is in a state where one person could be made better off without making anyone worse off. Most people would agree this is not a good state to be in, i.e. we can do better. 
That means a "pareto improvement" is possible. 

So what we would want is to in fact make that person better off (as no one else gets harmed and one person benefits). In trying to improve upon that state, we change it. We keep doing this until it is no longer possible to make anyone better off without making someone worse off. I.e. we stop improving when it's no longer possible to improve. 

So a state where pareto improvements are possible is not efficient since we can do better. Hence, states where no pareto improvements are possible (we can't do better) are the best possible ones. The best possible states are called efficient.

Hence, efficiency means a pareto improvement is no longer possible. In other words: it is not possible to make someome better off without making someone else worse off.