We need to know Pareto improvement to understand Pareto Optimality/Efficiency.
Pareto Improvement
If we can find a way to make some people better off without making anybody else worse off, we have a Pareto improvement.
Following from this, we can give the following definitions for Pareto Efficiency/ Pareto Optimality:
Pareto Efficiency/Optimality
If an allocation is such that no Pareto improvement is possible, then it is known as Pareto efficient/ Pareto optimal. The occurrence of such a situation is called Pareto Optimality/Efficiency.
Pareto Inefficiency
If allocation allows for a Pareto improvement, we have a Pareto inefficient allocation. The occurrence of this is, in turn, called Pareto Inefficiency.