Decompression is stopping at predetermined depths as you ascend to the surface for a predetermined amount of time. The time at each decompression stop depth is determined by how deep you go, what bottom gas you breathed, how long you stayed at depth, what decompression gasses you are using, the physical exertion of the dive, and your personal fitness level.
Decompression training involves learning about decompression procedures, decompression illness (DCI), decompression gasses, oxygen, decompression theory, decompression tables, and dive computers. When descending to deeper depths and staying for extended period of times, you enter the realm of decompression diving. This is no small element, as not properly following decompression procedures and requirements could end in causing you DCI or even death.
Many of the major training organizations offer decompression procedure courses. Training organizations such as NAUI, IANTD, GUE, TDI, and PADI all off different courses that are either directly focusing on decompression or build decompression procedures into the courses curriculum. However you decide to learn and understand decompression procedures, it’s extremely important that you do before attempting deep or long dives that require multiple levels of decompression. Proper training is imperative for safe execution and success.