A Knight of the Seven Kingdoms: What exactly is a hedge knight?

HBO's new Game of Thrones spin-off, A Knight of the Seven Kingdoms, features more knights than you can shake a lance at.

The House of the Dragon sequel takes place at a tournament (or "tourney") in The Reach's Ashford Meadow, where only knights are allowed to enter "the lists" — spaces for publicly demonstrating their combat and jousting skills. But what of Ser Duncan "Dunk" the Tall (Peter Claffey), who is constantly scorned as a "hedge knight" rather than a "true knight."

What exactly is a hedge knight, and why does it matter?