If you've ever used Twitter or Instagram that allows you to "mention" other users, you already know what this means.
But basically if your talking about the worlds re-known Turkey killer and want to mention Pat in your post so that it gets his attention instead of wondering if he'll see it, you use the @ symbol and then start typing his name. The forum will predict who your looking for based on what you type and it will give predetermined suggestions until you click one. And the end results looks like this
@coyote101 ...and he will get a notification.