A theme is a template that determines the way that your online store looks and feels. Different themes have different styles and layouts, and offer a different experience for your customers. For example, if you're selling spa products, then you probably want your online store to feel relaxed and luxurious. Alternatively, if you're selling electronics, then you might want your online store to look energetic and sleek.
When you make changes to your theme or switch to a new theme, it doesn't affect the other parts of your account. You can play around with different theme styles and settings without worrying about the rest of the content in your admin.