וַיַּ֨רְא אַהֲרֹ֜ן וְכָל־בְּנֵ֤י יִשְׂרָאֵל֙ אֶת־מֹשֶׁ֔ה וְהִנֵּ֥ה קָרַ֖ן ע֣וֹר פָּנָ֑יו וַיִּֽירְא֖וּ מִגֶּ֥שֶׁת אֵלָֽיו׃
און אַהרן און אַלע קינדער פֿון ישׂראל האָבן געזען משהן, ערשט די הױט פֿון זײַן פּנים שטראַלט, און זײ האָבן מוֹרא געהאַט צו גענענען צו אים.
Rashi
וייראו מגשת אליו AND THEY WERE AFRAID TO STEP NIGH UNTO HIM — Come and see how great is the power (influence) of sin! For before they streched forth their hand to sin what does Scripture say? (Exodus 24:17) “And the sight of the glory of the Lord was like devouring fire on the top of the mountain in the eyes of the children of Israel”, — and yet they were not afraid and did not tremble! But after they made the golden calf they recoiled and trembled even at the sight of the rays of glory of Moses! (Sifrei Bamidbar 1:8)
Chizkuni
וייראו מגשת אליו, “they were afraid to approach him.” According to the plain meaning of the verse when they beheld him, they thought that they were looking at an angel. (Compare Torah shleymah by Rabbi Menachem Kasher item 241 on this verse)
Da'at Zekenim
והנה קרן עור פניו, “and behold, the skin of his face emitted rays of light;” apparently his forehead reflected some of the brilliance of G–d’s “hand,” which protected him from getting a glimpse of G–d’s essence while he was standing in the cleft of the rock G–d had assigned to him for that purpose.