They are similar to ControlFiles and .changes files, in the syntax, but they have a different meaning. The first part of the file contains some information about the package that is usually derived from the ControlFile. The second part is a list of the files in the SourcePackage, and uses ?md5sums and ?signatures to try and ensure their integrity.
The usual way to generate a .dsc file is using dpkg-source though often scripts will create one for you when building (dpkg-dev does this). You can however create one by hand, or using another script if you need to. You can also edit the files if, for instance, one of the referenced files changes.
When looking for ?sponsorship the ?sponsor will often want to be given a link to this file, as with that information they should be able to find the rest of what they need. This means that you should generally include a link to it in any ?RFS mails, as this makes it easy for the ?sponsor to download your package using a tool like dget.