An anchor is an image, a specific memory, a song, a trinket, a pattern of behavior. It's something you use to pull your mind back to a defined state, in the pursuit of the zone it's often something that reminds you of being deeply relaxed and yet focused.
Here a brief article on it
Fran's post had a similar thread to it, using "something" (because it's specific) to get a person to relax into that state of mind in order to perform better.
It also happens on assembly lines when people are in effect hypnotized by their repetitive actions and lose touch with the world but perform their task with amazing efficiency because they are in their "zone" where the conscious mind, instead of judging right and wrong, or good and bad, takes a back seat and lets the body perform its actions without oversight.
I often start with
progressive muscle relaxation before I go out and combine it with self-hypnosis or guided meditation.
The image I use for the guided meditation won't be the same as yours. The idea is that once you have yourself completely at ease and fully relaxed you either see, smell, touch, hear, or taste something in that state that serves as your anchor. That anchor, as long as you keep it the same each time becomes a reminder for your mind of that state. The more you practice it, the easier it becomes to invoke the state of relaxation just by using that anchor.