The teacher was referring to Einstein's theory during the lecture.

referring 是动词 refer 的现在分词形式。refer 由前缀 re-（表示"再次"）和词根 fer（来自拉丁语 ferre，意为"携带"）组成，本义是"再次提及"或"指向"。在句中表示"提到、谈及"的意思。