Registry cleaning is almost totally unnecessary (unless you are getting "problems" with your computer when it MAY help).
Registry cleaning will not make your computer run any faster at all. Even if it does it will be by such a small amount you wont notice it.
The registry is a large file in Windows that is used to store information about the computer. Windows itself, and most programs you install, will update the registry and sometimes when you uninstall a program it leaves behind "orphan" things in the registry.
However these "orphan" things do no harm and do not slow down your computer.
Some registry cleaners, when they do a scan, report "150 errors" (or similar) in the registry. Dont believe them, they are just pretending there are errors to get you to pay for and use their product.
Most registry cleaners are a con.