Workflows that need to rerun periodically could naively be implemented as a big for loop with a sleep where the entire logic of the workflow is inside the body of the for loop. The problem with this approach is that the history for that workflow will keep growing to a point where it reaches the maximum size enforced by the service.
ContinueAsNew is the low level construct that enables implementing such workflows without the risk of failures down the road. The operation atomically completes the current execution and starts a new execution of the workflow with the same workflow Id. The new execution will not carry over any history from the old execution. To trigger this behavior, the workflow function should terminate by returning the special ContinueAsNewError error: