A SAX (Simple API for XML) parser is a particular mechanism for parsing XML documents. Using a SAX parser has the advantage over the DOM-based parser in that it is not necessary to build the explicit DOM tree. On the other hand, it can be difficult to build a SAX parser because it requires management of complex state.
Combining SAX parsing with a GNU Bison generated parser is appealing because it allows the Bison parser to manage all of the state. Additionally, the .y file encapsulates the equivalent of a DTD but in a much more readable form. The combination makes using SAX parsing a lot simpler.