Entropy is a measurement of how much usable energy there is, but; It is ALSO a measurement of how much unusable energy there is.
You are correct to be a little confused by the statement "entropy is a measurement of how much usable energy there is", because, even though true, it's not very accurate. It is however, how most sources seem to define the term.
Think of it as a plank balanced on a log. If a closed system starts with (hypothetically) 100% usable energy; we will symbolize this by piling 100 rocks on the left hand side. The rocks symbolize energy in the system. If they are on the left side of the plank they are usable, if they are on the right side, unusable.
As we use the plank-on-a-log's energy, some of the stones from the left side will start rolling to the right side. This is an increase in entropy. Say, 30 stones are now on the right side.
Now lets look at an identical plank that was made earlier. This one has 60 stones on the right side and 40 on the left. This planks entropy is higher than our previous one.
So we have several conclusions from this analogy.
1. As energy is used, entropy increases
2. As entropy increases, unusable energy % increases
3. As entropy increases, usable energy % decreases
4. As entropy decreases (in an open system), unusable energy % decreases
5. As entropy decreases (in an open system), usable energy % increases
I think a better way of defining it may be:
Entropy is: The ratio of usable and unusable energy in a given system. Higher entropy represents a greater % of unusable energy, where as Lower entropy represents a greater % of usable energy.