Which fields should be pre-filled with previously input information when there are form errors?

1. What does pre-fill mean?
2. How do I pre-fill an HTML form from a URL?

What does pre-fill mean?

(transitive) To fill in advance.

How do I pre-fill an HTML form from a URL?

Build Your Pre-fill URL​

To add a form field to the URL, we add a question mark ? after the form URL, then add the form field name, followed by an equal size = and the pre-filled value.