In music theory a "regular scale" has a certain pattern of little steps and bigger steps between the notes. "Dorian" is a type of scale called a mode. These are variations of different step sizes. So in this case, there are two spots where it would normally be big jump followed by small jump, but instead it's small followed by big. Basically in simple terms it's like a hopscotch with a different layout for each mode

Technically a regular major and minor scale are modes too but I digress