On the top right corner of the main page, you'll see either "pings" or "New Posts to you" (in red). When you just see "pings", that means you have already looked at the responses to you, when you see "New Posts to you" that means someone has responded to you since the last time you refreshed the main page.
That's a "ping". When you get the "New Posts to you" message. It's called a ping because it's a way of notifying someone that they have a response waiting. A ping list is a list of screen names some people have that they use to "ping" a large group of people at once, to notify them of something interesting in a thread. They post that list of names, separated by semi-colons, in the "To" box when they post a comment.
Hopefully that helps!
Most people only want to be jerks!