ARIA Described By can broadly be used to describe relationships. We have other tools in our pocket for giving a label to a field or other interactive object. We can also use ARIA Label, and that allows us to put text for the screen reader only into our code, or ARIA Labeled By. ARIA Labeled By takes an ID, so that is where something that’s visually on the page can be linked. Those things would replace the HTML label. The third option is this ARIA Described By, that will link it to the field, but that does not replace the label, it announces it as secondary. That also takes an ID.
Two places I would say ARIA Described By is used commonly would be if we had instructions for the field. Say we have a birthdate field, and under the field we have text that says MM/DD/YYYY. We would give that an ID, and give the text field ARIA Described By equals that ID. We would also use the same code to associate an error with the field that it’s describing.