I've been writing for about 11 months and during this whole time I've been working on a science fiction novel. It's probably the most meaningful hobby I feel I've ever undertaken and I'm extremely excited just to think about writing when I'm doing other things, it's great! I like to talk about my story with friends and family, and I'm just curious as to what the general consensus is about sharing your unfinished work. Should you worry that someone might steal your concepts/ideas?
I'm also a new author hoping to publish a fantasy series that I've been working on for a while now, along with some other projects. I worried about this a lot when I first started being active on social media platforms, blogs, and writing communities. The idea that my work could be stolen made me very hesitant to share it in the first place. But as time went on, I realized that it wasn't anything to be overly concerned about, especially since, as many of the responses here also state, sharing the general idea/concept of your book isn't the same as sharing an entire manuscript.
I would even go so far as to suggest that you could probably share excerpts and fun facts about your book with others as you're writing the book (though, if you're thinking of getting traditionally published, depending on the publisher's requirements, sharing excerpts might not be a good idea). In my experience it has helped keep readers engaged and interested in my stories! (But, at the same time, the writing circles in which I am active have proven themselves quite trustworthy, and I've never had issues with plagiarism or someone stealing my ideas.)
Alternatively, another way of sharing without really sharing is sharing excerpts from your book that are no longer viable (as in, you've changed that part of the book/plot and the excerpt you share is no longer relevant to your story, but it's fun thing to share with others).
But of course, as with anything to do with the internet, your primary concern should be your security, and you should be careful with your writing if you do intend to share it online. For example, one thing that I find myself doing is that if I have something special - a particular piece of worldbuilding, perhaps, or a juicy plot twist - that I don't want anyone to have the chance to steal, I simply won't share it. So really it's more of a balancing act, or it has been in my experience. At the end of the day, it's up to you to decide what you feel comfortable sharing online.