A flame next to a task can mean two different things: the task has a due date, or it is marked urgent.
A flame with a number or date: the due date
When a task has a due date, Marvin shows it next to the title. By default you see a flame with the number of days until it is due, like 3d. Hover over it to see the exact date.
You can change how the due date looks. Open ☰ → Settings and find Due date indicator. Under Style choose:
Short Days (3d): a flame and the number of days
Long Days (due in 3 days): written out, without a flame
Date (3 Apr): a flame and the date
Turn on Always red if you want due dates to always stand out in red.
A flame on its own: urgent
With the Urgency feature you can mark a task as urgent without giving it a fixed deadline. It is shown as a flame icon. Type *urgent or *fire in the task title, or set it from the task's menu. If you turn off simple mode in the Urgency settings, you get more levels, shown as more flames or a different color.
Changing the urgency icon
If the flame is confusing next to your due dates, you can change it. Open the Features page, find Urgency and open its settings. There you can:
choose the Icon Style: Filled, Lined or Emoji
click a level's icon to pick a different icon
click its color to change the color, or turn on Use theme color