Michael, that's checked yes by default, though you can uncheck it before posting something. It makes it so the post or comment you make will appear in the activity feed on the site's main/home page, visible to all who go to the home page. Having comments appear there makes it easy to see what been happening lately on the site when you just go check the main home page. :)
Those irritated by grain of sand best avoid beach.
-Strumelia proverb c.1990