The PPC represents the maximum possible output that an economy
can come out with at a certain point in time.
Points on PPC indicates that resources are used most efficiently
and thus the economy is producing at its maximum. Within the PPC,
resources are not used as efficiently. Unemployment can be
considered a type of inefficiency because it constitutes a resource
that is not fully used. Points outside the PPC are impossible for
an economy to achieve.
PPC shift outwards when there are more resources into the
economy in the long run (eg population growth, etc). It can also
shift outwards when technology makes the usage of resources more
efficient (eg computer). Shift inwards will be bad... cuz it will
represent the opposite
At A levels, the PPC curve you draw is normally for 2 variables
(simplified). A 3 variable curve will be 3D. In actual fact, there
are many more things an economy can produce.
For the simplified PPC that you draw, points on the PPC curve
line can also be viewed as an opportunity cost curve; they are
similar.
Hope it helps.