Adding an endpoint creates extra rewrite rules for each of the matching places specified by the provided bitmask. For example:
add_rewrite_endpoint( 'json', EP_PERMALINK | EP_PAGES );
will add a new rewrite rule ending with "json(/(.*))?/?$" for every permastruct that describes a permalink (post) or page. This is rewritten to "json=$match" where $match is the part of the URL matched by the endpoint regex (e.g. "foo" in "[permalink]/json/foo/").
A new query var with the same name as the endpoint will also be created.
When specifying $places ensure that you are using the EP_* constants (or a
combination of them using the bitwise OR operator) as their values are not
guaranteed to remain static (especially
Be sure to flush the rewrite rules - see flush_rewrite_rules() - when your plugin gets activated and deactivated.