regsvr32.exe is used to register DLLs and ActiveX controls (OCX) in the Windows registry. New entries (class IDs) are created in the registry database that refer to the corresponding DLL file. After that, the functions of these DLLs can be used by other modules.
Normally, this registration is done automatically by a setup program. In special cases or problems, you can also register the DLL yourself.
To do this, make sure that you are logged in as a user with administrator rights. Then run regsvr32 > .exe followed by the path and name of the DLL.
Tip: You can easily paste the path by dragging the DLL from a Windows Explorer window to the Run dialog.
For more information, please refer to Microsoft Support: Regsvr32 Usage and Error Messages