GUI Validation in Sharepoint

Hi ,

I came across something new in SharePoint "GUI Validation"

I want to ask that

What is actually GUI Validation in SharePoint

and

what are the ways/methods for providing GUI validation on any user's input , in SharePoint ??

 Can it only be done through OUT OF BOX ??What are the methods which can be used for providing validation ??

User's Input can be a document upload , list entries , site creation etc.. 

any help would be highly appreciable.

Thanks in advance