Generically, in audio and video, a "pre-roll" is material that comes before the start or edit point. In the context of this discussion, I assume that a "pre-roll" is an ad or teaser that plays before the main content.
The JW Player is a major player (pun intended) in the industry... so I see no harm in mentioning specifics... which can be configured for a variety of ad deployments. There is a JW Player module available for Drupal.
There is also a JW Player Support Community, with sections on servers and streaming, on using playlists, etc. I'm not sure what the WebmasterWorld Content Management forum's policy is with regard to linking to these, but they're readily findable.
I am not sure how to upload the files to it
It's likely that the JW Player requires a streaming server to seamlessly play a pre-roll ad followed by the main video... and that your hosting doesn't accommodate streaming video. I'm not the one to help you on this, but it looks like it's covered in the JW Player forums.