Complex sentence: The "acharonim" follow the "Rishonim", the "first ones"-the rabbinic scholars between the 13th and the 16th century following the Geonim and preceding the Shulkhan Arukh.
Split into simpler sentences: The "acharonim" follow the "Rishonim", the "first ones". The Rishonim are the Rabbinic scholars between the 13th and the 16th century following the Geonim and preceding the Shulkhan Arukh.