It is used to plan your decompression so you do not come up with excessive nitrogen in your blood therefore giving you the bends.
The tables give you time limits at different depths where most people will not get decompression sickness. There are different tables out there, but all have the same commonalities, such as having short time limits at deeper depths and longer time limits at shallower depths.
You may also want to see a longer answer in the question "What does decompression mean in diving?"
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