The Rebbe said this discourse on Shabbat Balak in 1970, just after the 12 Tammuz Lubavitch festival, celebrating the release of the Sixth Rebbe from imprisonment and exile in Soviet Russia, in 1927. He had been arrested because of his attempts to keep Judaism alive in the USSR.  The discourse explores the them of 'darkness' and 'light', and the fact that the light after darkness, or liberation after imprisonment, is felt more deeply than if it were light all the time.  This links with a discussion of the word 'Echad' (One) in the Shema:  this word expresses the idea that G-d (light) is being revealed in the world, which might be a world of darkness.