He always chats with waiters and waitresses and becomes their best friends during the course of dinner.

during 是介词，意思是"在...期间"，表示某个时间段内发生的事情。 course 名词，本义是"过程、进程"。 "the course of" 是固定搭配，由定冠词 the、名词 course 和介词 of 组成，意思是"X 的整个过程或时段"。 "during the course of dinner" 整体意思是"在晚餐期间"，相当于 during dinner，但更正式，强调从开始到结束的完整用餐时段。