With regard to a production possibilities curve, how is a point on the curve different from a point below the curve or a point above the curve?
I assume that you are asking about production possibilities curves/frontiers. I have edited your question to reflect this.
A production possibilities curve (PPC) shows what outcomes are efficient in a given economy. It shows what combinations of two kinds of products the economy can produce. In a PPC, the curve shows the maximum possible output. A point on the line is something the economy can achieve if it uses all of its resources as efficiently as possible. A point below the curve is inefficient. The economy could produce more than that if it were being run more efficiently. A point above the curve is impossible in the current circumstances. The economy would need more resources in order to achieve that outcome.