וַיַּ֨רְא אַהֲרֹ֜ן וְכָל־בְּנֵ֤י יִשְׂרָאֵל֙ אֶת־מֹשֶׁ֔ה וְהִנֵּ֥ה קָרַ֖ן ע֣וֹר פָּנָ֑יו וַיִּֽירְא֖וּ מִגֶּ֥שֶׁת אֵלָֽיו׃
And when Aaron and all the children of Israel saw Moses, behold, the skin of his face sent forth beams; and they were afraid to come nigh him.
Read kabbalistic (mystical) interpretation of Exodus 34:30: and classical mystical sources.