Someone who's dashing is stylish, with a certain bold, attractive spirit. Your dashing uncle might charm and impress your friends with his stories of travels around the world and his fashionable outfits.
Characters in old books are frequently dashing — adventurous and gallant, like a dashing hero on a white horse or a dashing pirate seeking his fortune at sea. The adjective dashing is almost always used to describe men, and for that reason it's a bit old-fashioned. In the 18th and 19th centuries "to cut a dash" meant "to act brilliantly" or "to give a showy appearance," and dashing came from this meaning.