Well, usually it's your followers. But if you have settings adjusted, it could be friends or a custom list of people you've selected. And keep in mind, if your profile is public, it's more accessible to a wider audience, though that doesn't mean everyone will necessarily see your stories.
TikTok doesn't allow users to know who has viewed their stories. This is to maintain some level of privacy and keep the experience more casual. So, unfortunately, you won't be able to find out who's been checking out your stories.