Over time, rechargeable batteries naturally deteriorate. With each cycle, a full charge and depletion, the maximum amount energy that a certain battery can store decreases. This is naturally a very gradual development and can only really be noticed after a few months of usage.
Different devices have different methods to find out the current battery health. On an android device, you can use the AccuBattery app to find out how much your device's battery has already deteriorated.
Charging to 100% deteriorates a battery much more than a slightly less full charge. If a 100% charge can be considered as able to cause one cycle of wear, a 91% charge would only cause 0.48 cycles of wear. So if you sacrifice a bit of battery life early on, you can exponentially lengthen the battery life of your device. A better example would be how a 50% charge would only cause 0.05 cycles of wear.