It means "quintessential D&D"; you have enough resources that the DM can throw lots of problems at you but not so many resources that every problem is assumed to be solved in favor of the players. In 5e, I'd say 5-12.
Once you get to around level 13, the difficult of the game stops becoming contingent on the imagination of the DM and becomes contingent of the imagination of whichever players are playing full-casters. "Mid-level" D&D represents a point where the scales are balanced the most closely, the DM has maybe a modicum more power than the players but it's razor-thin (the game does not function at all in a perfect balance between dm and players).