An authenticator is a way to prove to a computer system that you really are who you are (called authentication). It is either:
Authenticator tokens are common when one program needs to authenticate itself to a larger server or cloud repeatedly. For instance, you (the human) might sign on to a secure website with your name and password, after which you can surf around inside the secure server, visiting different web pages. Every time you move to a new page, however, the server must believe that you are the same person who originally signed in (otherwise it will refuse). Your browser keeps an authenticator token, which it sends upon every page request (often as a browser cookie), that does this.
More complex situations might involve a program that runs automatically (say, at 4:00am every morning) that similarly requires authentication to get at the data it needs, but there's no human around to log in for them. An authenticator token must be prepared in advance that this program uses. Ultimately, some human must authenticate to create such a token.