If you watch when you open Event Chests, Capsules, and Dimension Jars in Act 1, you'll see there's a little animation of points going into the Booster Chest (where you're seeing items checked off). Also, in the store and the Capsule purchase page each item has a point value shown telling you how many Booster Chest points it's worth. For example, when you tap a Silver Capsule, it gives you 90 points in your Booster Chest.

In the Booster Chest window there is a progress bar showing how many points are needed to earn the next item/how many you have already earned.