Candidate cards are cards that represent initiatives you're not quite sure where to drop into your roadmap. Think of them as projects you'd like to work on, but aren't sure where to add it into your roadmap without having to reprioritise it completely. It's a draft section for you to work with as things move along.
To add a new candidate card, simply click on the 'add card' button at the top of the column.
To move a candidate card to the roadmap, click on "Move to" > "Roadmap." This will add it under your 'Future' column first, and then you're free to move it to any column.