Common air is about 79% N2 and 20% O2 (the rest is trace amounts of other gasses.) As you breathe, your body can metabolize the O2 like normal, but the N2 accumulates in your body's tissues. As you ascend from the depths, the N2 comes out of solution and forms bubbles. These bubbles can enter your bloodstream, and if they reach places like your lungs or your brain, they can occlude blood vessels, causing problems. For the brain, it's similar to having a stroke (basically, it's exactly that) and for the lungs, it's basically a pulmonary embolus, which means gas exchange can't occur as normally happens when you breathe. (The bubbles are too big to pass through the capillary walls and be exhaled.) The bubbles that form in other spaces, like joints, can cause excruciating pain. You increase in pressure by one atmosphere per every 33 feet of sea water, so being down at 100 feet, the guy was breathing air that was about 4x as dense as on the surface -- that means he's breathing 4x more molecules of oxygen and nitrogen. To resolve this, he'd need to come up to the surface amazingly slowly, and take stops at progressively shorter distances and for exponentially longer times. The stop that would be, say, ten feet from the surface might be half a day in length. All so the nitrogen can come out of solution and be exhaled in a controlled fashion.

To make this easier, they commonly put people into a pressurized chamber at depth, haul it out of the water, and then decompress on the surface, eliminating variables like changes in buoyancy, predatory animals, and the like. They can also help speed things along by tweaking the balance of oxygen to nitrogen.