Okay kiddo, do you know what a doodle is? It's when you draw something funny or silly just for fun. Well, drolleries are like fancy doodles that people used to draw in old books a really long time ago.
When people were making books by hand back then, they would decorate the pages with all sorts of pictures and designs. Some of these drawings were serious, like pictures of important people or scenes from the Bible. But sometimes the artists would let their imaginations run wild and draw all sorts of silly creatures and funny little scenes. These silly drawings are called drolleries.
Sometimes these drolleries were hidden away in the corners of the pages so you had to search for them like a little treasure. Other times they were right in the middle of a page and would make you giggle.
So drolleries are really just silly drawings that people would make in books a long time ago. They were like little jokes that the artists would sneak in to make people smile.