Ask HN: Best explanation or tutorial of Kanban?

People swear by Trello and Kanban-style project management seems to be preferred. In the Heinlein sense of the word, I don't seem to "grok" Kanban, so I think I must be missing something. I'm not sure how columns of cards really helps manage a project, rather than the Gantt concept, which is "this stage" takes "this long". Can someone point me towards a good resource that explains how Kanban truly manages projects and why it's popular?