You can flag an issue to indicate that it's important. Flagged issues (cards) are displayed in yellow (in both Work mode and Plan mode), with the "flag" icon replacing the Priority icon:
Flagging or unflagging an issue
Click the Agile link's down-arrow in the top navigation bar, then select your preferred board from the resulting dropdown menu.
- Click either Plan or Work.
- Click the issue key (or type t in Work mode) to display the Issue Detail View on the right-hand side of the screen.
- Select Add flag / Remove flag from the 'cog' drop-down.
You can close the Issue Detail View by clicking the 'x' icon (or typing 't').