It probably just means "no T-shirts, no worn or faded jeans, no worn out chinos, no sneakers."

But if it's a nice restaurant: better jeans with a dark wash. Like AG. Or J Crew if you're on a budget.

Or chinos, as long as they are well made - like Incotex, or something lower priced like Ralph Lauren slim fit.

Solid or striped button down shirt. Point collar. Blue or white preferred. Avoid oxford cloth, unless it's pinpoint oxford.

Navy blazer. "Smart" looking blazers are single-vented, made with Italian wool and you'll not find one under $600 or $700.

A pair of drivers like Tod's or Ferragamo. Wear socks if your feet are not tan.