Complex sentence: Jantina Hellingmolen is what the Dutch describe as an "achtkante beltmolen," an eight-sided smock mill which is built into a mound providing access to the sails.
Split into simpler sentences: Jantina Hellingmolen is what the Dutch describe as an "achtkante beltmolen". This means an eight sided smock mill which is built into a mound which provides access to the sails.