Rashi on Deuteronomy 4:6/רש"י על דברים ד׳:ו׳

1ושמרתם AND YE SHALL BE ON THE WATCH [TO DO] — This refers to the study of the laws (Sifrei Devarim 58),

2ועשיתם — this must be explained according to what it implies: AND YE SHALL DO.

3כי היא חכמתכם ובינתכם וגו׳ FOR THIS IS YOUR WISDOM AND UNDERSTANDING [IN THE EYES OF THE PEOPLES] — i.e. through this ye will be accounted wise and understanding men in the eyes of the peoples.