The sentence in the picture above is a verbal one. It starts with the past فعل ماضي verb كـــَـــتـــَــــبَ (wrote). Now, who wrote? Muhammad is the subject or doer فــاعــِـــل of the action. He (Muhammad) is the one or the person who wrote. So, Muhammad wrote, but wrote what? He (Muhammad) wrote the lesson الــدَّرسَ . Hence, the lesson is the object  الــمــَــفـــعــــُـــول  به. It is the thing that received the action. The action (writing) happened to the lesson.