It's where you can install a different version of the phone's operating system to Samsung OneUI.
If you are interested, you should do a lot of research as there are risks including bricking your phone or installing a 3rd party piece of software which is potentially malicious or is not maintained to the same level of security as OneUI.
You may have problems with some banking apps and potentially some may say it is against their terms and conditions.
Personal view of mine is that I would only take official Samsung software through the official download channel or something from the Google Play / Samsung Store.