LDH (more current name = LD) stands for lactic dehydrogenase. It is an enzyme that is found in a great many types of cells. Several of these cell types have LD's that have slight differences in their chemical makeup and are called isoenzymes.
When cells are damaged or die, they release their enzymes into the blood stream. By measuring the various enzymes in the blood, it possible to determine which tissue has been damaged and even how long ago that damage began.
Changes in LD levels have been shown to be reasonably sensitive "reflections" of what is going on at a cell level so increases in LD are taken to mean that some tissue somewhere is experiencing stress. If the LD isoenzyme is from the heart - then you know about heart disease. If the LD isoenzyme is from the liver - then you know there is liver trauma. And, if the LD is from lymphocytes, then you know that there is increased activity in the lymphocytes.