It's supposed to be transmitted orally, like a story from Homer or something, so I won't copy and paste it here, but you can find it easily enough by searching. Basically, without quoting it word-for-word, it's old bluesman advice from the "diddly-bow" era that when starting out on guitar, string it with one string only and play it until you can provoke emotion from an audience with one string. When you can do that, add another, repeat the cycle, add another string, until all six strings are on the guitar.