The acharonim follow "the" Rishonim "-the first ones", the Rabbinic scholars living between the 13th and the 16th century, and preceeding the Shulkhan Arukh.
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 preceeding the Shulkhan Arukh.