It was to gather a reasonably large number of escaped slaves, arm them, and then retreat into the Appalachians, where they would slowly bleed the institution of slavery dry through raids. Maybe sounds crazy to modern ears, but it had already been done before: en.wikipedia.org/wiki/Maroons

Maroons - Wikipedia

I would say, with the benefit of hindsight, that Brown lived right on the tail end of when something like this would be possible. Industrialization would mean that the settled parts of the US would easily dominate subsistence farmers/hunters in the hills. But that wasn't obviously true at the time.