Its a song that was mentioned on a teacher web page that discussed ways of using the book "Night John" with students. As you know, the central premise of the story is that all slaves are forbidden to learn to read; one of the reasons is of course, that they could use this skill to help themselves escape.
The song "follow the drinking gourd" illustrates how knowledge was passed in other ways; according to the website I visited, the song contains directions on how to head north, following along particular landmarks at particular times of year.
In case you want to visit the website, I'll insert the web link here: link
And here is the weblink where you can listen to the tune of the song:
I hope they work! (I'm crossing my fingers!!)