Buckets (or roadmap cards) are high level areas in themselves. The ones that are in the current term you can usually think of as releases, which contain multiple ideas. The ones that are in the future are placeholders. They represent a problem that you want to solve or are a placeholder for that one idea you have is placed. The roadmap cards in the future column will have less details, maybe with no ideas attached to them. As you move them to the Near Term and Current columns, they'll start containing more information, ideas, and user stories.
ProdPad is meant to be flexible and keeps these different ways of working in mind. It doesn't try to tie you down to a specific process, because we know you end up working in a fluid way. Instead of thinking of the roadmap as a release plan, think of it as a strategic communication document that outlines the big steps you're going to take or the problems you're trying to solve to meet your product vision.