"The market equilibrium outcome is Pareto efficient." What is meant by this?

"No matter whether price starts out above or below its equilibrium value, a mutually beneficial transaction will always be possible...the equilibrium price and quantity constitute the best outcome attainable, given the initial attributes and endowments of buyers and sellers."

I am not agreeing with the answer as I think Pareto efficient means there is no way of improve ones situation without harming someone else,