Rashi on Exodus 19:12/רש"י על שמות י״ט:י״ב

1והגבלת AND THOU SHALT SET BOUNDS — set boundary marks for them as a sign that they should not approach the mountain from the boundary and beyond.

2לאמר SAYING — The boundary says to them, “Take care not to ascend from here and further on” and you, too, admonish them regarding this.

3ונגע בקצהו OR TOUCH THE EXTREMITY THEREOF — i. e. even the extremity thereof.