The Progression mechanic is a mechanic of where the game shows you your progress or the development of some sort. For example, a building time of an object or your character development. So an experience bar.
In this game the experience bar is labeled a bit better and it includes numbers and the percentage. (Neon green bar)
On the PlayStation when you earn trophies, you level up your profile and also it shows you the current percentage of your account and it also shows the current percentage of the trophies earned in a game.
In my own game I would like to include the progression bar for when it comes to leveling up, and also a progression bar to show the game completion rate.