Gravitas is seriousness and dignity. It's hard to take someone seriously who's wearing full clown makeup, no matter how much gravitas they speak with.
Gravitas is a Latin word that means "weight or heaviness." It came to mean a figurative weight after gravity acquired a primarily scientific meaning. A biography of Abraham Lincoln will inevitably be full of gravitas, and places like libraries, museums, and university buildings seem to have gravitas, or dignity, while grocery stores and gyms usually do not.