Answer: the number of apples used for cider.
Step-by-step explanation:
If 100 apples are shipped then out of an initial quantity X of apples, there are only left:

Out of this x-100 apples 1/3 are used for making cider, and 2/3 are used for desserts.
When we multiply 1/3 by (x-100) we will get the number of apples used for cider.
And when we multiply 2/3 by (x-100) we will get the number of apples used for desserts.
In this case we are asked for what does the following expression represent:

This, for the reason previously stated, is the number of apples used for cider.