In a nutshell, Foner wants Sanders to connect his political ideas to the radical tradition in the United States rather than to present-day socialism in Scandinavia (Denmark, Norway, etc.). By doing so, socialism will not seem so foreign to Americans.
Here is a taste:
You could begin with Tom Paine and other American revolutionaries who strove not simply for independence from Britain but to free the new nation from the social and economic inequalities of Europe. Embrace the tradition of abolitionists, black and white, men and women like William Lloyd Garrison, Frederick Douglass, and Abby Kelley, who, against overwhelming odds, broke through the conspiracy of silence of the two major parties on the issue of slavery and helped to create a public sentiment that led to Lincoln’s election and emancipation. (And don’t forget to mention that slaves represented by far the largest concentration of wealth in the United States on the eve of the Civil War, that slaveholders were the richest Americans of their time, and that nothing could be accomplished without confronting their economic and political power.) Refer to the long struggle for women’s rights, which demanded not only the vote but also equality for women in all realms of life and in doing so challenged some of the most powerful entrenched interests in the country.