פּירוש אויף שמות 34:33

ויתן על פניו מסוה AND HE PUT A VEIL ON HIS FACE — Render it (מסוה) as the Targum does: בית אפי, a cover for his face. מסוה is an Aramaic expression (from the root סוי, “to look”). It occurs in the Talmud (Ketubot 62b) “her heart perceived (סּוי), and again in Ketubot 60a “הוה קא מסוה לאפה”, where מסוה is an expression for looking: “he looked into her face”, i. e. he gazed at her. Here, too, מסוה denotes a cloth that was put in front of the face and of the region of the eyes. And out of reverence for the “rays of glory” — that not everybody should feast himself on them — Moses used to put the veil in front of them (the eyes), but took it off during the time when he spoke to Israel, and during the time when the Omnipresent conversed with him until the moment when he was going out and also when he went out he went out without the veil.

ויכל משה מדבר אתם, “When Moses had finished speaking with them, etc,”

“And he put a veil on his face.” Since everyone was looking at him, and Moses was extremely humble and shy, he was embarrassed that they were looking at the radiance of his face’s skin, for this is the nature of anyone who is shy. Therefore, he put a veil on his face to cover the eyes, but when he came before God to learn Torah from His mouth, then he needed to remove the veil of embarrassment from his face, because "the shy person does not learn" (Avot 2:6). For this reason it says, and when Moses came before God to speak with Him, he would remove the veil until he went out.