The progress field shows the percentage of linked cards that have a specific status.
For example: on a project card with attached tasks card it can show how many of the task cards have the status completed.
This helps in getting a quick overview of the status of underlying cards when looking at the broader picture.
Start by opening the card type
Next add a field of type "Progress"
You are asked on what linked card type should be monitored
Finally you set the filter that determines when a card is "completed"
The last step determines the number of cards that are displayed as a % of the total number of linked cards of that type.