The Production Possibilities Curve, shows the maximum combinations of two goods a theoretical economy can produce with the current state of technology and given the available resources.
Any increase in the production of one commodity must be done at the expense of the other, the opportunity cost of the good increased is the number of unit of the other that we have to give up.
in this question the opportunity cost of producing 10 more unit of good X is the 5 units of good Y that was given up.
