Production Possibility Fronteir... Oh the days :P
It shows the maximum optimal output of the economy, given under the assumption that all the variable inputting factors are operating at maximum efficiency. This relates to the Factors of Production (Land, Labour, Capital & Enterprise).
The PPF = LRAS essentially, showing the maximum possible output a company is able to produce at.
Because the PPF shows the maximum, let's take two variables to simplify what it's trying to show.
Let's assume an entire economy only produces motorbikes and cars. We can associate the X axis with Cars and the Y axis with motorbikes. Now the PPF is showing what the economy can produce with all its factors of production employed.. thus we can assume, if we are to produce at Point C on the curve we will produce C1 cars and C2 motorbikes, as illustrated.
Now.. if an economy realises there's more money to be made by producing motorbikes, they may decide to distribute the available factors of production towards motorbike manufacturing. This is illustrated by the movement from C->B on the curve. Thus an economy will now only produce B1 amount of cars but B2 amount of motorbikes. I think by now you understand the foundational concept.
To produce more of one thing, you have to give up the other if all factors are operating at their maximum efficiency. This shows an opportunity cost, as whoever runs this particular economy would have to make a decision as to whether to maintain production at current levels or produce more of one thing in sacrifice of another.
It resembles scarcity in the fact that there's limited input. There are only a finite number of resources (factors of production: Land, Labour, Capital, Enterprise) in which to work with. So allocating those resources efficiently is vital to the success of an economy.
The trade-off in this instance would be.. should we manufacture more cars? more motorbikes? or maintain current levels?
If you want I can briefly explain how the PPF can shift out, as we just talked about movement along the curves for your question.
If there's anything more you require explaining, don't hesititate to hit me up