Reading the docs…
Stopped.
An action item is something someone will do: a row with a checkbox, an owner and a due date. Press A and click the board to add one, then type what needs doing. Select stickies and press ⌥ ⇧ A (Action items in the bar) to turn them into action items in place.
Click an item’s chips to set them:
The left edge turns amber when an item is due within three days or today, and red once it’s late, where the due chip says how many days late it is. Done items are struck through.
The Issue chip links the item to an issue: type #123 (in the board’s usual repository), owner/repo#123, or paste a link. Or stage a new issue in a repository, assigned to the item’s owner, with the due date and a link back to the board; once you push, the item is linked to it.
Once it’s linked, the item’s status is the issue’s: it’s done when the issue is closed. Ticking it stages closing the issue (completed); ticking it again before you push takes that back. An item linked to an issue you can’t read stays open as far as you can see.
Open action items belong in the next session. Select two or more and choose Carry over (⌥ O), or use Carry over… in the Facilitate menu for all the open ones, and pick one of your boards. They go into its Actions section (or a new “Carried over from…” section), with their owners, due dates and issues, each saying where it came from. Carrying over twice doesn’t double up.
When you make the next retro from a template, Bring open action items from does the same as it’s made.