What does it mean when a chef says a dish is a "rustic" dish eg. a "rustic pot roast"
There's no standard definition - it's a descriptive term, not a standardized culinary one. In practice, it's often used in fine-dining contexts to denote a deliberately less subtle or carefully assembled dish. Stews are often considered "rustic", in contrast to highly precise cuisine like sushi. A loaf of irregular sourdough bread might be called "rustic" in contrast to a croissant. It's often meant to imply a focus on flavor, simplicity, and familiarity rather than on appearance, originality, or technique.

It's a somewhat disingenuous term in that it's often used by and applied to professionals who focus overall on fine cuisine. It implies something that could have been made with much finer precision and presentation, but was kept deliberately rough. A Michelin-starred chef's chili con carne might be considered "rustic"; your grandmother's chili made with ground beef and tomato paste probably wouldn't be. A roadside barbeque shack's brisket isn't necessarily "rustic", but the same beef cooked and plated in a high-end restaurant would be.

That all said, it bears repeating that this isn't a consistently applied term. It's used in the same way as words like "classic" or "artisanal" - because it sounds good to diners and conveys an impression of simple quality, not because it means anything specific in regards to preparation.