Specifies the maximum amount of time a resource will be considered fresh. Contrary to Expires, this directive is relative to the time of the request.
In other words this is time interval for which any client such as browser or proxy server might use cached version.
How exactly it affects google I'm not sure. Googlebot might take it into account in some way (but I doubt they blindly trust you). This might be an issue if you have it on your main page because the bot might not come back for 3 days (259200 seconds = 3 days) to see new articles/posts. The same goes for new comments. Still if google ignores your site for much longer than that, the issue is not with caching but somewhere else.