Yokel is a disparaging name for someone from a small town or the countryside. To call someone a yokel is to imply that they are unsophisticated, uneducated, and probably dim-witted.
Before the age of internet and television, if you grew up in a small town or on a farm, you probably wouldn’t have known about the latest fashions of Paris and Milan or the best restaurants in New York. Even now, if you live in a remote place, you may have a somewhat provincial understanding of the world. If so, when you visit the Big City an unkind “city slicker” may call you a yokel.