The pre-roll video will play anytime someone goes to the player on your stream, whether the stream is online or offline. It plays right when someone logs on, then it will "join" the stream. If the video is offline, it will go to your offline video selections. If your stream is live, it will join the stream right where you're at. It will play every time someone goes to the player, so if they log out, then log back in, the video will play for them again.
If you want a video that will play before your stream actually begins, you'll need to include that in the content that you're streaming, and simply start your stream earlier than your event time to give it time to play.
You can set up the pre-roll video on your stream under the Options button.