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 )".