If you're not a REST expert, you probably use the same HTTP codes over and over in your responses, mostly 200, 404, and 500. If using authentication, you might perhaps add 401 and 403; if using redirects 301 and 302, that might be all. But the range of possible status codes is much broader than that and can improve semantics a lot. While many discussions about REST focus on entities and methods, using the correct response status codes can make your API stand out.
201: Created
Many applications allow creating entities: accounts, orders, what have you. In general, one uses HTTP status code 200 is used, and that's good enough. However, the 201 code is more specific and fits better: