Not in Extra = no places left, that's all. Unis get oversubscribed because they're seen as 'elite', or because they're very popular, or because they are small, or because they give fewer offers per place. Exeter is in a nice place, and is a good uni, so it isn't surprising that it's popular, I suppose. Also, some unis go into Extra without having many/any places, and only make Extra offers to international students.
It could mean you're more likely to be let in, because they have fewer offers per place. Who knows? Probably best to try to get your grades!