The grammar of a language defines its syntax. Parsers (e.g. Marpa) take a sequence of bytes, characters or tokens, check if it conforms to the grammar, and assigns meaning to the components of the sequence as per the grammar.
For example, the job of the parser is to receive "a+b*c" and return "a multiplication of ( an addition of ( identifier a ) and (identifier b ) ) and ( identifier c )".
As I say, tuits in the round are the issue and without the time to cook up some working example I'd just be glossing the various documentation and BNF/ParseRec stuffage. I think a simple example can be cooked up but it's also not entirely trivial or my forte (hint, hint to others who might have something to show). This is quite interesting and I've been waiting to try some Marpa though. If I can grab an hour tonight I'll revisit this otherwise, the weekend, otherwise... uh, what were we talking about?