CPU Credit regulates the amount CPU burst of an instance. You can spend this CPU Credit to increase the CPU performance during the Burst period. Suppose you are operating the instance at 100% of CPU performance for 5 minutes, you will spend 5(i.e. 5*1.0) CPU Credit. Similarly if you run an instance at 50% CPU performance for 5 minutes you will spend 2.5(i.e. 5*0.5) CPU Credits.
CPU Credit Balance is simply the amount of CPU Credit available in your account at any moment.