Dovetailing is a kind of joint . . . a way of joining two pieces of wood together. Shaped like a trapezoid, with the largest part furthest inside, so you can't just pull it out again.
And a mill is for grinding things — there's a two stones together, and the top has a little hole in it. If you put grain in there, as the water turns the wheel, the wheel turns the mill, and turns the grain into flour.