A timely reminder is a prompt or notice that serves as a helpful and relevant reminder, given at an appropriate moment. It could be in reference to something important such as a task, appointment, deadline, or responsibility, and it's intended to serve as a gentle nudge or a friendly reminder to take care of the matter before it becomes too late or forgotten. The phrase implies that the reminder is given at just the right time when it can be most effective in helping someone remember what they need to do next.