Exodus 34:33-35 (ESV), 33 “And when Moses had finished speaking with them (the Israelites), he put a veil over his face. 34 Whenever Moses went in before the LORD to speak with him, he would remove the veil, until he came out. And when he came out and told the people of Israel what he was commanded, 35 the people of Israel would see the face of Moses, that the skin of Moses’ face was shining. And Moses would put the veil over his face again, until he went in to speak with him.”
The Apostle Paul explains this in 2 Corinthians 3:7-18 (NIV), 7 “Now if the ministry (the Law Covenant) that brought death, which was engraved in letters on stone, came with glory, so that the Israelites could not look steadily at the face of Moses because of its glory, transitory though it was, 8 will not the ministry of the Spirit be even more glorious? 9 If the ministry that brought condemnation was glorious, how much more glorious is the ministry (the New Covenant in Christ’s reign) that brings righteousness! 10 For what was glorious has no glory now in comparison with the surpassing glory. 11 And if what was transitory came with glory, how much greater is the glory of that which lasts! 12 Therefore, since we have such a hope, we are very bold. 13 We are not like Moses, who would put a veil over his face to prevent the Israelites from seeing the end of what was passing away (the end of the Law Covenant as pictured by Moses’ face glowing). 14 But their minds were made dull, for to this day the same veil remains when the old covenant is read. It has not been removed, because only in Christ is it taken away. 15 Even to this day when Moses is read, a veil covers their hearts. 16 But whenever anyone turns to the Lord, the veil is taken away. 17 Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. 18 And we all, who with unveiled faces contemplate the Lord’s glory, are being transformed into his image with ever-increasing glory, which comes from the Lord, who is the Spirit.”