Rav Yechezkel of Kuzmir zy”a says that the Torah is teaching us that when a person wants to learn, he should not get distracted by disturbances that get in his way. On the contrary, by overcoming these obstacles to learning, one is able to better serve Hashem. He compares this to a reflection which is made by blocking one’s hand with a mirror (mareh), and which enables a person to see himself.
Accordingly, the pasuk is saying that Hashem shows Himself “in a mareh”, meaning that He appears at times of distraction and enables a person to overcome.