Gases dissolve in water. Since humans are mostly water, this means that the human tissues hold quite a bit of dissolved nitrogen from the atmosphere. Also, greater the pressure, greater the amount of gas that will dissolve. For people mainly living on the surface of the Earth this is not a problem, but once someone enters an environment of greater pressure (usually underwater: the pressure increases by one atmoshphere every 10 meters), the water in their body becomes more willing to accept nitrogen, or rather the environment becomes more capable at cramming the nitrogen into them.
Even this wouldn't be a problem, but sooner or later the person's tissues contain so much dissolved nitrogen that it would bubble if they ascended immediately to the surface. Having this much dissolved gas in the tissues is called having an decompression obligation. This is considered a bad thing in recreational diving, since if something goes wrong in the diver's SCUBA gear and they must perform an emergency ascent, they have a good chance of getting the decompression illness a.k.a. the Bends.
So, divers who routinely develop a decompression obligation on their dives must ascend according to a precalculated decompression schedule which details a series of decompression stops at specific depths. They must be prepared to deal with all problems underwater and to follow the schedule no matter what. This means good training, suitable gear and a good team.
Preventing drowning in an out-of-air situation by ignoring your decompression obligation may sometimes be good idea, but if the nitrogen loading was considerable, the results may not be pleasant.