I am really confused over PKCS. I saw this question but I do not have enough reputation to leave a comment so I am asking a new question to clarify.

I am trying to understand what exactly PKCS and in extension what CMS is. I have stumbled on the line from the RFC mentioned in the aforementioned question.

This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content.

How can a syntax be used to digitally sign stuff?!! I can understand if it states it is a syntax used to describe digitally signed stuff...but signing? how can a syntax be used to sign?

This all tie into my confusion with PKCS/CMS and x509. I understand that x509 is a standard that describes what should make up a certificate, how is this different from PKCS/CMS?