It scans the registry for errors
You can do a registry scan by downloading a program called ccleaner & which is free
Yes it can speed up your pc
Basically, the registry is where all vital information for applications and files is gathered. Usually, the registry is quite organized, allowing the computer to acquire any information it needs quickly and without hassle. Although, when the registry starts becoming overloaded with information, which is caused by constant user activity, errors are going to accumulate, period. The more errors the registry has, the more your PC will suffer.