You don't ... per se send one. That's where you get various notifications regarding your activity, and the way people interacted with your activity. For example I have just hit the like button on your original post, and under that bell icon you will receive an alert saying something like "Maddox reacted to yout post in thread "name of thread" with like ". If you like any of my posts in a thread I will get an alert. If somebody quotes you in a thread you get an alert, if you get a private message you get an alert, so on and so forth and it works the other way around when you interact with other peoples' posts. Makes sens?