Alerts are visual cues to draw the eye to logic-based data points on a dashboard / chart. They can be used to inform the audience of the nature of the measures in view, or to highlight data points above / below a specified threshold, in order to alert the audience to a potential need to investigate the underlying cause.
First explores the use of ASCII symbols in order to create the required shape to use as the alert indicator.
The second example shows alerts using some custom shapes.
The original blog posts containing those two videos from Andy Kriebel are here:
Another example from Ryan Sleeper is linked below. Here, Ryan demonstrates a couple of different uses for alerts, including dynamic labelling and alerting banners.
Plus another of Andy’s Tableau Public workbooks also includes the use of alerts: