A Registry Cleaner is a software tool designed to scan and fix issues within the Windows operating system's registry. The registry is a database that stores settings and options for the operating system and installed applications. Over time, it can accumulate errors, such as broken links or outdated entries, which may slow down system performance. By using a registry cleaner, users can remove these unnecessary or harmful entries, potentially improving their computer's speed and stability. However, it's important to use such tools cautiously, as incorrect changes to the registry can lead to system problems. Always back up the registry before making any modifications.