Adam Clarke on Exodus 19:23

The people cannot come up - Either because they had been so solemnly forbidden that they would not dare, with the penalty of instant death before their eyes, to transgress the Divine command; or the bounds which were set about the mount were such as rendered their passing them physically impossible.

And sanctify it - וקדשהי vekiddashio. Here the word קדש kadash is taken in its proper literal sense, signifying the separating of a thing, person or place, from all profane or common uses, and devoting it to sacred purposes.