‘And Yahweh came down on Mount Sinai, to the mountain peak, and Yahweh called Moses to the mountain peak, and Moses went up. And Yahweh said to Moses, “Go down. Charge the people lest they break through to Yahweh to gaze and many of them perish.”
As God looked down He knew the hearts of the people, and He had mercy on them. He knew that their trembling fear (Exodus 19:16) was slowly turning into awed curiosity (‘to gaze' - Exodus 19:21), and that in such a state some might become careless and thus perish. So He sent for Moses to come up to Him on the mountain peak, and from there He sent him down to prevent it happening. He did not want His gracious appearance to turn into tragedy. The people would be filled with awe to think that Moses had been allowed into the holy mount while God was resident.
“Charge the people.” They were to be given a strict charge not a mild admonishment. The matter was deadly serious.