Error production in parser-tools/yacc

I'm trying to learn how to use the package parser-tools-lib.

There is one thing which I would like to understand but I'm not able to figure it out.

These point in the docs

An error production can be defined by providing a production of the form (error α), where α is a string of grammar symbols, possibly empty.

I'm not sure how to use this error production, if possible at all. I suspect it is not the same thing as

(error expr)
The expr should evaluate to a function which will be executed for its side-effect whenever the parser encounters an error.

Is there any tutorial, additional documentation, examples that I can check out?