What is the significance of mild reversible ischemia?

Coronary narrowing. Ischemia (which is found on a stress test) connotes heart muscle not getting enough blood flow to meet its needs. Mild ischemia implies that the defect showing up on stress test is more subtle, i.e. That there is slightly less blood flow to the area during stress than at rest. It usually means the severity of underling narrowing is less, though by definition it is still enough to limit flow.
Poor perfusion. Ischemia is caused by low or inadequate blood flow and oxygen delivery to a tissue: heart attack stroke amputation renal failure are sequellae of ischemia but mild is mild reversible means it was treated and improved.