RegInOut is a "registry cleaner" tool.
These have been around for over 20+ years (since Windows 95 moved to the "registry" from INF files), reached a pinnacle in 2011, and began to decline in recent years (typically due to the rise of mobile).
The idea that using a "registry cleaner" tool would speed up your PC, fix errors and generally give your system the vitality that is required in the modern age, is a rare commodity. Unfortunately, most "registry cleaner" tools are nothing more than bloatware.
RegInOut has been around since 2009, where it was released on Softpedia.
Since then, it has grown thanks to a combination of article/content marketing and continual upgrades. Despite it being at version 5, the service is still predominantly desktop-oriented, meaning that it has NO web based contingent.
Typically, whenever you're looking at using a repair tool for Windows - you need to ensure that it's not actually going to cause any damage to the underlying system. This means that you should be looking for either continual updates (which RegInOut actually seems to have) OR a definition-based scanning system (similar to how an antivirus tool works).
With the modern world moving even further to a "web-first" platform, it's apparent that RegInOut's lack of web contingent have placed it behind the likes of CCleaner (which has CCleaner Cloud) - but, as mentioned, this does not necessarily mean the system is inherently worse.
RegInOut has 4 main features - System Scan, Game Boost, Registry Defrag, Startup Speedup. These are typical of most "registry cleaner" tools.
The way the system works is to manage the underlying settings that Windows uses to operate. This means that if you use the "System Scan" option, you're basically letting the system clean out any of the errors that it identifies in the registry - or with the software - of your system.
If you use the "Game Boost" feature - it basically means that RegInOut will close down any unnecessary "services" that may be running in the background. Things like the print spooler, antivirus and other systems that are simply not required if you're playing a game.
Finally, if you use the "Startup Speedup" or "Registry Defrag" features - you're basically looking at cleaning out any ingrained issues inside the Windows system itself. Such things as having problems within the registry or faulty applications causing your computer to take a lot longer to boot up.
The most important thing to look for with any "registry cleaner" tool is whether it's been designed by a legitimate company. Sorcim - the author of RegInOut - is legitimate and seems to be committed to putting out high quality software.
Whilst I'm not partial to the effectiveness of its system, alongside CCleaner - which is undoubtedly the most trusted & respected of all the "registry cleaner" systems - RegInOut is certainly reliable and effective. Its latest version is 5.6.2.