When something is implemented, it's put in place or enforced. If you hear a rumor at work that all employees must wear hats with propellers, you'll probably be relieved to learn that it's not an implemented policy — at least, not yet.
The Latin word implementum means "filling up" or "fulfillment." A decision, plan, or policy is like an empty idea until it's implemented. An implemented rule, on the other hand, is in full force, and you'll have to stick to it. When a law has been suggested, officially proposed, voted on, and finally written into the law books, you can describe it as implemented.