Unity relies heavily on GUIDs to represent files and folders in your Unity project.
There are situations when you might have content / packages that you are loading into a project with conflicting GUID files (for example, multiple packages from a single Asset Store vendor).
This utility was created to help work around these issues by allowing you to regenerate the GUID files in your project.
Note: this is an ALL OR NOTHING action. You can NOT generate GUIDS for only a single folder (for example) or file.
If you don't know what the above means, then you don't need to do this.
Simply go to Assets -> Cleanup -> Regenerate GUIDs. This will show you the following prompt:
READ THE MESSAGE and then click 'Regenerate GUIDs' to continue.