Freediving is a sport where people try to hold their breaths and dive as deep under the water as they can with nothing to help them to breathe. Competitive freediving has many variations, mostly related to what equipment you can use. In some you can use masks, fins, or weights, while in others those are prohibited. The one thing that ties all freediving together is some kind of breath holding.
The world record for breath holding is 9 minutes and 2 seconds for women and 11 minutes 35 seconds for men. However, freedivers are moving and going very deep underwater so they hold their breaths for less time.
Freediving has a reputation for being very dangerous. However, if it's done right, the sport should not be all that dangerous. Most free divers who have died or had serious accidents have done so while training not at competitions with judges and safety officials.
The maximum depth in any kind of freediving is 300 meters for men and 257 meters for women. The depth a free diver can go changes though depending on the type of freediving. Whether or not you can use fins, weights etc. all impact how deep a free diver can go.
Freediving is an ultimate test of a human's ability. People do it to see how far they can push themselves and challenge themselves. Lots of freedivers say they achieve extreme calm while doing it. Freediving definitely isn't for everyone but if you want to push yourself to your human limits it might be the sport for you.