well it seems fairly straightforward to me. I would interpret it as meaning for example not being subject to EU laws, not being part of a deeply undemocratic and top-down kind of institution. More broadly I think there's a widespread feeling of dispossession, being ignored, powerlessness that, in the present climate, gets expressed as (or lumped in with) 'sovereignty'. These are the sorts of impressions I have got from hearing what people say about it. Obviously I can't speak for whoever you've been talking to.