Super Chest is a bonus chest with the best prizes on the Black Market. You can get it after opening a certain amount of regular chests. The progress bar is expressed as a percentage and it shows how close you are to it:
Depending on the chest type (Bronze, Silver, Golden) the progress bar is filed differently. The more valuable chests you open, the faster it fills. Once the bar is fully filled, you will be able to open a Super Chest, after which the bar resets.
You need approximately 8,500 keys to open a Super Chest.