It means that the supply of blood (and thus oxygen) is not sufficient during the maximum stress test, but that during rest it is (so the effect is reversible).
The position where the heart blood supply is sometimes not sufficient is part of the wall of the left ventricle.
The left ventricle is the part of the heart that queezes out the blood from the heart to the whole body. The pumping efficiency of the left ventricle may suffer from the lack of blood supply.