When we build an application using Visual Studio that time it will build only those files which have changed/modified. The files which are not changed/modified Will not build.
The clean solution are use to delete all the compiled DLLs and EXE files from bin/obj application directories.
When we Rebuild an application using Visual Studio that time deleted all complied like DLLs and EXE files and after that build all the files which are changed/modified or not.
It is a combination of both clean and Build. i.e. (CLEAN + BUILD)