Albert Barnes Commentary Exodus 34:10

"And he said, Behold, I make a covenant: before all thy people I will do marvels, such as have not been wrought in all the earth, nor in any nation; and all the people among which thou art shall see the work of Jehovah; for it is a terrible thing that I do with thee." — Exodus 34:10 (ASV)

Marvels - Explained in the following verse. Compare 2 Samuel 7:23; Psalms 77:14.