Answer
A rolling forecast is a continuously updated projection that always covers a fixed future horizon (e.g., the next 12 or 18 months). As each period ends, a new period is added and estimates are revised with the latest information, so the forecast never expires at year-end and always looks the same distance ahead.
Real-world example Each month-end the team drops the completed month and adds a new one, keeping a constant 12-month view.