As you descend, the higher pressure drives nitrogen from your breathing gas into your tissues. The deeper and longer you stay, the more nitrogen you absorb.
The no-decompression limit (NDL) is the longest you can stay at a given depth and still ascend directly to the surface without mandatory decompression stops. Past it, you must make staged stops to let nitrogen come out of solution gradually.
Ascend slowly
On the way up, dissolved nitrogen comes back out of your tissues. Ascend too fast and it forms bubbles instead of being exhaled — the cause of decompression sickness.
Keep your ascent to about 9–10 metres per minute (slower than your smallest bubbles) and make a 3-minute safety stop around 5 m on every dive, even when you're well inside your NDL.
Use the planner
The Dive Plaza dive planner models this with the Bühlmann ZH-L16C algorithm — it shows your NDL, any required stops, and a safety stop for the profile you enter.