Something that is out of bounds is something in which you are not allowed to enter. For example, in a restaurant, the kitchen is usually out of bounds to the customers. But "bound" also means "hop" or "bounce". A kangaroo will normally bound across the desert or the countryside and do whatever kangaroos do. But here, the kangaroo is so tired (exhausted) that he has no more bounce (or bounds) in him. He can't move anymore, so he is "out of bounds".

out of breath out of bounds What does a kangaroo do?